Stirred Batch Reactor Experiment

Introduction

A chemical reactor is defined as a device properly designed to let reactions occur under controlled conditions toward specified products. A batch reactor may be described as a vessel in which chemicals are placed to react. They are used in small-scale laboratory set-ups to study the kinetics of chemical reactions. A typical stirred batch reactor is shown below. It consists of a tank with an agitator and integral heating/cooling system. They are usually fabricated in steel, stainless steel, glass lined steel, glass, or exotic clay.

Chemical reactors may strongly differ in dimensions and structure; nevertheless, in order to derive a mathematical model for their quantitative description, essentially two major features are to be considered: the mode of operation and the quality of mixing. Classification according to the mode of operator can be ‘Continuous’ or ‘Discontinuous’. The discontinuous mode is that shown in Figure 1 above. Based on the quality of mixing, it can be perfect mixing or no mixing.
In homogeneous reaction systems, reaction rates depend on the concentration of the reactants. The specific effect of the concentration changes of each reacting species in a reaction system has to be determined by experimental methods because the balanced equation for the net reaction does not indicate how the reaction rate is affected by a change in concentration of reactants.
To determine the order and the reaction rate constant of a chemical reaction, variation of a property of the reaction mixture is observed as the reaction progresses. Such properties include concentration of a component, total volume of the system, electrical conductivity, refractive index, and so on.

The general form of the rate law is

r = k[A]m[B]n

Where
r is the reaction rate
K is the reaction rate constant
[A], [B] are molar concentrations of reactants
m, n are appropriate powers (orders) based on experiment data

In this experiment, the order and the reaction rate constant for the liquid reaction of caustic soda and ethyl acetate in a stirred batch reactor is determined. The balanced equation for the reaction between caustic soda and ethyl acetate is given as follows:

NaOH + CH3COOC2H5 → CH3COONa + C2H5OH

Objective(s) of the Experiment

     This experiment is carried out to determine the order and the reaction rate constant for the liquid reaction of caustic soda and ethyl acetate in a stirred batch reactor.

Equipments and Materials Needed

  1. Batch Reactor
  2. 0.1 M Sodium Hydroxide (0.5 litres)
  3. 0.1 M ethyl acetate (0.5 litres)

Procedures

1. Open the Armfield software and choose “Isothermal Operation”. Set up the Hot Water Circulator and adjust settings on the PID loop for Isothermal Operation and set point to 25oC.
2. Charge the batch reactor with 0.5 litres of the sodium hydroxide solution. Set the reactor agitator to 50% and press “Power on” to start the agitation. Then press “Hot Water Circulation” to start recirculating the water through the jacket reactor and vessel.
3. Carefully add to the reactor 0.5 litre of the ethyl acetate solution and instigate the data logger program. By recording the conductivity of the reactor contents with respect to time, the amount of conversion can be calculated.
Collection of data will be until a stable condition is reached in the reactor and this takes approximately 30 minutes.
4. Repeat the experiment with the temperature controller set on 30oC., 35oC., 40oC. and 50oC..
5. Repeat the experiment with the stirrer speed set on 30%, 40%, 60% and 70%.
6. Run the experiment using different initial concentrations of the reagents of 0.08M and 0.2M. 0.5 litre of each solution will be required for each mixture tested.

Results and Calculations

Having recorded the conductivity of the contents of the reactor over the period of the reaction, the conductivity measurements must now be translated into degree of conversion of the constituents. Both sodium hydroxide and sodium acetate contribute conductance to the reaction solution at the same molarity and temperature and a relationship has established allowing conversion to be inferred from conductivity.
Transfer the readings of conductivity with time to a computer spreadsheet as two columns of data.

Enter the following known constants from the experiment (ensure to use correct units):

  • aµ = (Sodium hydroxide concentration in the feed bottle)
  • ao = (Sodium hydroxide concentration in the mixed feed) =
  • bµ = (Ethyl acetate concentration in the feed bottle)
  • ao = (Ethyl acetate concentration in the mixed feed) =
  • T = (Reactor temperature)
  • V = (Total volume in the reactor vessel)

Using the spreadsheet, calculate the values of c, a, Ac∞, Aao, Ao, Aa∞, and A from the following formulae:

c = bofor bo < ao
c = aofor bo ≥ ao
a = bofor ao < bo
a = (ao – bo)for ao ≥ bo
Ac∞ = 0.070 [1 + 0.0284(T – 294)] cfor T ≥ 294
Aao = 0.195 [1 + 0.0184(T – 294)] aofor T ≥ 294
Ao = Aaoassumes co = 0
Aa∞ = 0.195[1 + 0.0184(T – 294)]aif a ≠ 0
A = Ac∞ + Aa∞

From the values of parameters calculated above, the spreadsheet can be used to calculated the values of sodium hydroxide concentration (a1), sodium acetate concentration (c1), degree of conversion of sodium hydroxide (Xa) and degree of conversion of sodium acetate (Xc) for each of the samples of conductivity taken over the period of the experiment using the following equations:

The concentration of NaOH can then be ploted against time. The specific rate constant, k is the slope of the plot of (ao-a1)/(ao-a1 ) against time. The effect of temperature on the reaction rate constant is determined through the Arrhenius law:

Where
ko is the frequency factor, E is the activation energy,
R is the ideal gases constant and
T is the absolute temperature.

Once the kinetic constant at three different temperatures is known, the frequency factor and the activation energy values can be computed from the plot of ln k against 1⁄T.
A logarithmic plot of reaction rate against concentration level for each of the reactants tested will give a straight line of slope equal to the power of the relationship.

References

  1. Wikipedia. ‘Batch Reactor’ https://en.m.wikipedia.org/wiki/Batch_reactor. Assessed on July 15, 2018.
  2. ‘Stirred Batch Reactor Experiment’. Department of Chemical Engineering, Obafemi Awolowo University, CHE 409 Practical.
  3. F. Caccavale et al. (2011). ‘Control and Monitoring of Chemical Batch Reactors, Advances in Industrial Control’. DOI 10.1007/978-0-85729-195-0_2, Springer-Verlag London Limited.

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