INTRODUCTION: The term 'modeling' refers to the development of mathematical equations which can satisfactorily explain and predict the behavior of a given equipment or system. The design equations used for plug flow reactor (PFR) and backmix reactor (BMR) represent two simple "models"corresponding to a longitudinal mixing of zero and 100 percent respectively. The behaviour of many practical reactors is approximated by one or other of these simple models. There are however several reactors which cannot easily be described by the PFR or the BMR model. Some of them exhibit only minor deviations from plug flow behaviour and are generally represented by a so-called "Dispersion model". The characteristic constant in the dispersion model (i.e. the model parameter) is called dispersion number (ND). The flow through equipment can be characterized depending on its dispersion number as follows:
ND = 0 Ideal plug flow
0 < ND < 0.002 predominantly plug-flow 0.002 < ND < 0.2 Intermediate flow 0.2 < ND < predominantly back mix reactor ND Ideal backmix reactor AIM OF THE EXPERIMENT: The aim of the present experiment is to determine the dispersion number for a given packed bed reactor at specific conditions of operation using the so called "stimulus response" technique (also called the "tracer technique"). METHOD: The method consists in establishing a steady flow of water through the given packed bed and then suddenly injecting a small quantity of tracer (e.g.Glacial Acetic Acid) into the incoming stream of water by means of a hypodermic syringe. The water exiting from the reactor is sampled at successive intervals of time and analyzed for its tracer content. MATERIALS REQUIRED: Glacial acetic acid .. . . . . . . . . 5 ml. Sodium hydroxide aqueous (0.1N) . . . . . . 250 ml. 50ml Beakers . . . . . . . . . . . 20 No. Hypodermic syringe (5ml) . . . . . .. 1 No. Also measuring cylinders (250 ml and 500 ml); stopcock; titration setup; standard oxalic acid (0.1n); and phenolphthalein indicator. PROCEDURE: 1. Study the packedbed unit and draw a neat sketch of the entire set up. 2. Allow water to flow through the packed bed section at a steady rate keeping a rotameter reading of 70. Measure the flow rate by means of the 250ml measuring jar and stop clock. If the flow rate is very much less than 300ml per minute increase the flow rate to a rotameter reading of 80. 3. Maintain the flow steadily for about 1 minute so as to allow the unit to attain steady state. Ensure that the bed is free from entrapped air bubble as far as possible. Measure the flow rate of water once again at this stage with maximum accuracy. 4. Number the 50-ml beakers serially and keep them in a tray. Keep this tray and the stop clock handy by the side of the unit. 5. Draw acetic acid into the syringe and flush the needle and the syringe of any bubbles. Retaining precisely 3.0 ml of acetic acid in the syringe purge out the excess acid through the needle. 6. Carefully handling the syringe introduce its needle into the inlet rubber tube. Now inject all the 3.0ml of aceticacid as fast as possible into the tube and simultaneously start the stop clock. 7. Collect samples of liquid exiting from the packed bed in the serially numbered beakers at the interval of 15seconds. Collect the first sample 15 seconds after injection of tracer. See that the volume of each sample is at least 15ml so as to suffice for volumetric analysis. 8. After all the 20 samples are collected stop the flow of water. Carefully drain the water contained in the packed column only into a 500ml-measuring jar and note its volume as accurately as possible. 9. Standardize the NaOH against oxalic acid. 10. Titrate 10-ml samples from each of the 20 beakers against NaOH using phenolphthalein as indicator. PRECAUTIONS: (1) Watch the rotameter continuously and regulate flow strictly constant. (2) Measure the hold up and flow rate with maximum possible accuracy. READINGANDCALCULATIONS Standardization of NaOH: Oxalic acid taken: Volume = Normality = NaOH titre Volume = Normality = Calculation of mean residence time ( ) and variance and variance ( ) (Complete the entries in columns 1 to 5 of the table on the next page) ( ) = = = PLOTTING OF GRAPH: Hold up volume V = ml. moles of acetic acid used as tracer, m = Reference concentration Co = 1000 m / V = gm - mole / litre (Complete the entries in columns 6 to 8 of the table. draw a graph of E versus ). EVALUATION OF DISPERSION NUMBER (ND) For values of < 0.02, use the formula : ND = /2 If > 0.02, evaluate ND by trial and error using the equation:
= 2 ND - 2
Hence the flow is:
Predominantly plug flow / intermediate / predominantly backmix.
(Strike out inapplicable terms)
CONSISTENCY TEST:
Rotameter reading =
Volumetric flowrate of water F = ml / min.
Space time, = V/F = min.
Compare the value of with that of and report the percent discrepancy.
SELECTED QUESTIONS:
1. Evaluate the total area under the curve (E) versus and compare the value with unity (Consistency test).
2. Plot (E) versus ( ) for an ideal PFR and an ideal BMR and compare the experimental curve with these ideal curves.
REFERENCES:
Levenspiel "Chemical Reaction Engineering " (2ed).
Sample No. Time (t) min Na0H titre value,ml (V) Vt Vt2 Tracer conc.,gm-mole/litre
E=C/Co
o o o (assumed) o o o o o
1
2
3
4
5
6
7
8
.
.
.
.
.
19
20
ND = 0 Ideal plug flow
0 < ND < 0.002 predominantly plug-flow 0.002 < ND < 0.2 Intermediate flow 0.2 < ND < predominantly back mix reactor ND Ideal backmix reactor AIM OF THE EXPERIMENT: The aim of the present experiment is to determine the dispersion number for a given packed bed reactor at specific conditions of operation using the so called "stimulus response" technique (also called the "tracer technique"). METHOD: The method consists in establishing a steady flow of water through the given packed bed and then suddenly injecting a small quantity of tracer (e.g.Glacial Acetic Acid) into the incoming stream of water by means of a hypodermic syringe. The water exiting from the reactor is sampled at successive intervals of time and analyzed for its tracer content. MATERIALS REQUIRED: Glacial acetic acid .. . . . . . . . . 5 ml. Sodium hydroxide aqueous (0.1N) . . . . . . 250 ml. 50ml Beakers . . . . . . . . . . . 20 No. Hypodermic syringe (5ml) . . . . . .. 1 No. Also measuring cylinders (250 ml and 500 ml); stopcock; titration setup; standard oxalic acid (0.1n); and phenolphthalein indicator. PROCEDURE: 1. Study the packedbed unit and draw a neat sketch of the entire set up. 2. Allow water to flow through the packed bed section at a steady rate keeping a rotameter reading of 70. Measure the flow rate by means of the 250ml measuring jar and stop clock. If the flow rate is very much less than 300ml per minute increase the flow rate to a rotameter reading of 80. 3. Maintain the flow steadily for about 1 minute so as to allow the unit to attain steady state. Ensure that the bed is free from entrapped air bubble as far as possible. Measure the flow rate of water once again at this stage with maximum accuracy. 4. Number the 50-ml beakers serially and keep them in a tray. Keep this tray and the stop clock handy by the side of the unit. 5. Draw acetic acid into the syringe and flush the needle and the syringe of any bubbles. Retaining precisely 3.0 ml of acetic acid in the syringe purge out the excess acid through the needle. 6. Carefully handling the syringe introduce its needle into the inlet rubber tube. Now inject all the 3.0ml of aceticacid as fast as possible into the tube and simultaneously start the stop clock. 7. Collect samples of liquid exiting from the packed bed in the serially numbered beakers at the interval of 15seconds. Collect the first sample 15 seconds after injection of tracer. See that the volume of each sample is at least 15ml so as to suffice for volumetric analysis. 8. After all the 20 samples are collected stop the flow of water. Carefully drain the water contained in the packed column only into a 500ml-measuring jar and note its volume as accurately as possible. 9. Standardize the NaOH against oxalic acid. 10. Titrate 10-ml samples from each of the 20 beakers against NaOH using phenolphthalein as indicator. PRECAUTIONS: (1) Watch the rotameter continuously and regulate flow strictly constant. (2) Measure the hold up and flow rate with maximum possible accuracy. READINGANDCALCULATIONS Standardization of NaOH: Oxalic acid taken: Volume = Normality = NaOH titre Volume = Normality = Calculation of mean residence time ( ) and variance and variance ( ) (Complete the entries in columns 1 to 5 of the table on the next page) ( ) = = = PLOTTING OF GRAPH: Hold up volume V = ml. moles of acetic acid used as tracer, m = Reference concentration Co = 1000 m / V = gm - mole / litre (Complete the entries in columns 6 to 8 of the table. draw a graph of E versus ). EVALUATION OF DISPERSION NUMBER (ND) For values of < 0.02, use the formula : ND = /2 If > 0.02, evaluate ND by trial and error using the equation:
= 2 ND - 2
Hence the flow is:
Predominantly plug flow / intermediate / predominantly backmix.
(Strike out inapplicable terms)
CONSISTENCY TEST:
Rotameter reading =
Volumetric flowrate of water F = ml / min.
Space time, = V/F = min.
Compare the value of with that of and report the percent discrepancy.
SELECTED QUESTIONS:
1. Evaluate the total area under the curve (E) versus and compare the value with unity (Consistency test).
2. Plot (E) versus ( ) for an ideal PFR and an ideal BMR and compare the experimental curve with these ideal curves.
REFERENCES:
Levenspiel "Chemical Reaction Engineering " (2ed).
Sample No. Time (t) min Na0H titre value,ml (V) Vt Vt2 Tracer conc.,gm-mole/litre
E=C/Co
o o o (assumed) o o o o o
1
2
3
4
5
6
7
8
.
.
.
.
.
19
20
