INTRODUCTION: The continuous stirred tank reactor (CSTR) is also known as a back-mix reactor and sometimes as a mixed flow reactor. Unlike in the tubular flow reactor a deliberate attempt is made in the CSTR to promote mixing to maximum extent possible.
CSTR's are not generally used for homogeneous reactions. However homogeneous reactions are most convenient for the study of CSTR behaviour.
MATERIALS REQUIRED:
Ethyl acetate (Aqueous) 0.1 N : 20 litres
Sodium hydroxide (Aqueous) 0.1N : 20 litres
Hydrochloric acid (aqueous) 0.05 N : 250 ml
Oxalic acid (standard) 0.1 N : 100 ml
FIDDLING WITH THE APPARATUS:
Switch on the magnetic stirrer and check that it is working. Similarly check that the control switch for speed variation of the magnetic stirrer is working. What speed would you like to choose for carrying out the experiment?
Start the flow of Na0H Wait and see if bubbling starts in the overhead tank. Does the manometer show sensitivity when you operate the stopcock controlling the Na0H flow. Repeat these tests with EtAc also.
What flow rates of the reactants would be suitable in your opinion for the Experiment?
KEEPING CONSTANT FLOW RATES:
Maintain a constant flow of Na0H reagent through the apparatus. Measure the flow directly by means of a measuring cylinder and stop watch when there is bubbling in the tank. Repeat the flow measurement and check the constancy of the flow within experimental error. What is the percent fluctuation in the flow rate about its mean value? What is the manometer reading? Without disturbing the Na0H flow rate establish the flow of EtAc reagent and measure its flowrate twice when there is bubbling in the overhead tank. Report percentage variation in flow and also the manometer reading. Keep the stirrer on at a convenient speed throughout the experiment.
IS THERE ACCUMULATION IN THE REACTOR:
Establish the flow of liquid from the reactor to the outside from the lowest of the three side nozzles. Measure the outflow from the reactor atleast twice. Do you conclude from the measurements that there is any accumulation or depletion of material in the reactor. What is the observed descrepancy between the inlet and outlet flow rates. If possible try to improve on your measurements so as to reduce the descrepancy.
DOES THE OUTLET CONCENTRATION VARY WITH TIME?
Take exactly 40 ml of 0.05N HCl in a measuring jar. Collect 20 to 25 ml of the reactor effluent directly in this measuring jar and gently shake it to cause quick mixing for the each addition of 1 ml. Note the exact volume of the sample collected. Transfer the contents of the measuring jar into a 250 ml conical flask. Rinse the measuring jar with a little distilled water and add the rinsing into the conical flask. Back titrate the contents with 0.05N Na0H using Phenolphthalein as indicator. Wait for about five minutes. Analyze another sample from the reactor in the same manner as before. Do you find any variation in the effluent concentration?. Keep maintaining steady flow through the reactor until two concentration values coincide. What is the concentration of the reactor effluent at this time? (When steady state is reached).
HOW TO DETERMINE THE VALUE OF REACTION RATE CONSTANT?
Close the bottom most side outlet nozzle of the reactor and arrange for outflow through the next higher side nozzle. Keep measuring the effluent concentrations at intervals of 5 minutes until a steady value is attained. Has the reactor took more time or less time to attain steady state as compared to the previous run?
Repeat the experiment raising the holdup of the reactor still further.
CLOSING DOWN:
Stop the flow of both reactants and stop the magnetic stirrer. Note down the hold-up volumes corresponding to the three levels in the reactor. Note the normality of standard oxalic acid. Standardize the Na0H in the tank and the sodium hydroxide used for back titration with standard oxalic acid. Standardize the HCl against the Na0H used for back titration.
METHOD OF CALCULATION:
Obtain concentration of Na0H() using the formula
CA = (V1 N1 – V3 N3) / V2 gm – mole / litre
Where V1 = volume of HCl, V3 = titre value and V2 = volume of sample collected, N1 and N3 are respective normalities. Calculate the feed concentration (CA0 and CB0) using the formula
CA0 = FANA / (FA + FB) gm – mole / litre
CB0 = FBNB / (FA + FB) gm – mole / litre
Where FA and FB are the flow rates of Na0H and EtAc reagents, and NA is taken to be 0.1 N. The normality NA is obtained from standardisations.
Obtain CB using the formula: CB = (CB0 – CA0) + CA gm - mole / litre
Obtain reaction rate from the formula: rA = F (CA0 - CA) / V gm – mole / (litre) (min)
Plot rA Vs CA CB and measure the slope. Report k in units of litre / gm – mole.min at the ambient temperature.
READINGS AND CALCULATIONS
Preliminary Titrations:
1) Determination of Na0H concentration:
Volume of Oxalic acid taken = 10 ml
Normality of oxalic acid = (given)
Volume of Na0H run down = ml
Normality of Na0H in feed tank ( ) =
Normality of Na0H used for back titration (N3) =
2) Standardisation of HCl:
Volume of HCl taken = 10ml
Volume of Na0H run down = ml
Normality of HCl (N1) =
OUTLET CONCENTRATION (CA) of Na0H:
_______________________________________________________________________________
Run No.
1 2 3 4
-----------------------------------------------------------------------------------------------------------------------
Flow rate (FA ml/min)
Flow rate (FB ml/min)
Vol of HCl taken (V1 ml)
Vol. of sample collected (V2 ml)
Vol. of Na0H rundown in back
titration (V3 ml)
Gm-moles of Na0H rundown
= gm-mole of HCl used up
= V3 N3
Gm-moles of Na0H in sample
= gm-moles of HCl used up
= (V1 N1 - V3 N3) gm - mole/lit
CA = (V1 N1 – V3 N3) / V2
OUTLET CONCENTRATION (CB) of Et Ac.
Na0H concentration in feed
CA0 = FA /(FA + FB) gm-mole/lit
Et Ac concentration in feed
CB0 = FB (0.1) / (FA + FB)
CB = (CB0 - CA0) + CA ____________________________________________________________________________
EVALUATION OF k:
______________________________________________________________________________
Run No.1 Run No.2 Run No.3
----------------------------------------------------------------------------------------------------------------------
Volume of reaction mixture
in the reactor 160ml 265ml 375ml
Total flow rate (F lit/min)
Rate of reaction rA
= F (CA0 – CA ) / V g.mole/(lit)(min)
CA CB …. …
Plot rA Vs CACB and report the value of reaction rate constant.
k = --------------------------------lit / (Gm-mole)(min) at…………..0C
SELECTED QUESTIONS:
1. Assuming the reaction rate constant to be 9 lit/(g.mole)(min) calculate CA for the four runs in your experiment. Comment on the validity of the backmix model for the equipment used.
2. The following non-idealities may occur in a practical mixed reactor:
a) by-pass or short circuiting
b) deadwater regions
suggest the different methods by which these non-idealities can be reduced.
3. It is assumed in the experiment that steady state conditions prevail after about 20
minutes. How can you experimentally check whether steady state is attained or not at an
instant of time in a given mixed flow assembly.
4. It is immaterial for purpose of experiment whether you start the reactor initially filled with
distilled water or with a mixture of reactants. Why?
CSTR's are not generally used for homogeneous reactions. However homogeneous reactions are most convenient for the study of CSTR behaviour.
MATERIALS REQUIRED:
Ethyl acetate (Aqueous) 0.1 N : 20 litres
Sodium hydroxide (Aqueous) 0.1N : 20 litres
Hydrochloric acid (aqueous) 0.05 N : 250 ml
Oxalic acid (standard) 0.1 N : 100 ml
FIDDLING WITH THE APPARATUS:
Switch on the magnetic stirrer and check that it is working. Similarly check that the control switch for speed variation of the magnetic stirrer is working. What speed would you like to choose for carrying out the experiment?
Start the flow of Na0H Wait and see if bubbling starts in the overhead tank. Does the manometer show sensitivity when you operate the stopcock controlling the Na0H flow. Repeat these tests with EtAc also.
What flow rates of the reactants would be suitable in your opinion for the Experiment?
KEEPING CONSTANT FLOW RATES:
Maintain a constant flow of Na0H reagent through the apparatus. Measure the flow directly by means of a measuring cylinder and stop watch when there is bubbling in the tank. Repeat the flow measurement and check the constancy of the flow within experimental error. What is the percent fluctuation in the flow rate about its mean value? What is the manometer reading? Without disturbing the Na0H flow rate establish the flow of EtAc reagent and measure its flowrate twice when there is bubbling in the overhead tank. Report percentage variation in flow and also the manometer reading. Keep the stirrer on at a convenient speed throughout the experiment.
IS THERE ACCUMULATION IN THE REACTOR:
Establish the flow of liquid from the reactor to the outside from the lowest of the three side nozzles. Measure the outflow from the reactor atleast twice. Do you conclude from the measurements that there is any accumulation or depletion of material in the reactor. What is the observed descrepancy between the inlet and outlet flow rates. If possible try to improve on your measurements so as to reduce the descrepancy.
DOES THE OUTLET CONCENTRATION VARY WITH TIME?
Take exactly 40 ml of 0.05N HCl in a measuring jar. Collect 20 to 25 ml of the reactor effluent directly in this measuring jar and gently shake it to cause quick mixing for the each addition of 1 ml. Note the exact volume of the sample collected. Transfer the contents of the measuring jar into a 250 ml conical flask. Rinse the measuring jar with a little distilled water and add the rinsing into the conical flask. Back titrate the contents with 0.05N Na0H using Phenolphthalein as indicator. Wait for about five minutes. Analyze another sample from the reactor in the same manner as before. Do you find any variation in the effluent concentration?. Keep maintaining steady flow through the reactor until two concentration values coincide. What is the concentration of the reactor effluent at this time? (When steady state is reached).
HOW TO DETERMINE THE VALUE OF REACTION RATE CONSTANT?
Close the bottom most side outlet nozzle of the reactor and arrange for outflow through the next higher side nozzle. Keep measuring the effluent concentrations at intervals of 5 minutes until a steady value is attained. Has the reactor took more time or less time to attain steady state as compared to the previous run?
Repeat the experiment raising the holdup of the reactor still further.
CLOSING DOWN:
Stop the flow of both reactants and stop the magnetic stirrer. Note down the hold-up volumes corresponding to the three levels in the reactor. Note the normality of standard oxalic acid. Standardize the Na0H in the tank and the sodium hydroxide used for back titration with standard oxalic acid. Standardize the HCl against the Na0H used for back titration.
METHOD OF CALCULATION:
Obtain concentration of Na0H() using the formula
CA = (V1 N1 – V3 N3) / V2 gm – mole / litre
Where V1 = volume of HCl, V3 = titre value and V2 = volume of sample collected, N1 and N3 are respective normalities. Calculate the feed concentration (CA0 and CB0) using the formula
CA0 = FANA / (FA + FB) gm – mole / litre
CB0 = FBNB / (FA + FB) gm – mole / litre
Where FA and FB are the flow rates of Na0H and EtAc reagents, and NA is taken to be 0.1 N. The normality NA is obtained from standardisations.
Obtain CB using the formula: CB = (CB0 – CA0) + CA gm - mole / litre
Obtain reaction rate from the formula: rA = F (CA0 - CA) / V gm – mole / (litre) (min)
Plot rA Vs CA CB and measure the slope. Report k in units of litre / gm – mole.min at the ambient temperature.
READINGS AND CALCULATIONS
Preliminary Titrations:
1) Determination of Na0H concentration:
Volume of Oxalic acid taken = 10 ml
Normality of oxalic acid = (given)
Volume of Na0H run down = ml
Normality of Na0H in feed tank ( ) =
Normality of Na0H used for back titration (N3) =
2) Standardisation of HCl:
Volume of HCl taken = 10ml
Volume of Na0H run down = ml
Normality of HCl (N1) =
OUTLET CONCENTRATION (CA) of Na0H:
_______________________________________________________________________________
Run No.
1 2 3 4
-----------------------------------------------------------------------------------------------------------------------
Flow rate (FA ml/min)
Flow rate (FB ml/min)
Vol of HCl taken (V1 ml)
Vol. of sample collected (V2 ml)
Vol. of Na0H rundown in back
titration (V3 ml)
Gm-moles of Na0H rundown
= gm-mole of HCl used up
= V3 N3
Gm-moles of Na0H in sample
= gm-moles of HCl used up
= (V1 N1 - V3 N3) gm - mole/lit
CA = (V1 N1 – V3 N3) / V2
OUTLET CONCENTRATION (CB) of Et Ac.
Na0H concentration in feed
CA0 = FA /(FA + FB) gm-mole/lit
Et Ac concentration in feed
CB0 = FB (0.1) / (FA + FB)
CB = (CB0 - CA0) + CA ____________________________________________________________________________
EVALUATION OF k:
______________________________________________________________________________
Run No.1 Run No.2 Run No.3
----------------------------------------------------------------------------------------------------------------------
Volume of reaction mixture
in the reactor 160ml 265ml 375ml
Total flow rate (F lit/min)
Rate of reaction rA
= F (CA0 – CA ) / V g.mole/(lit)(min)
CA CB …. …
Plot rA Vs CACB and report the value of reaction rate constant.
k = --------------------------------lit / (Gm-mole)(min) at…………..0C
SELECTED QUESTIONS:
1. Assuming the reaction rate constant to be 9 lit/(g.mole)(min) calculate CA for the four runs in your experiment. Comment on the validity of the backmix model for the equipment used.
2. The following non-idealities may occur in a practical mixed reactor:
a) by-pass or short circuiting
b) deadwater regions
suggest the different methods by which these non-idealities can be reduced.
3. It is assumed in the experiment that steady state conditions prevail after about 20
minutes. How can you experimentally check whether steady state is attained or not at an
instant of time in a given mixed flow assembly.
4. It is immaterial for purpose of experiment whether you start the reactor initially filled with
distilled water or with a mixture of reactants. Why?
