EFFECT OF MIXING IN A STIRRED TANK REACTOR

ESSENSE OF THE PROJECT

To study the performance of a Stirred Tank Reactor

using different parameters.

To design a better and a controlled mixing process

that utilizes raw materials and avoids pollution.

To cut down the mixing expenditure.

MIXING

• Mixing is defined as the reduction of in-homogeneity in order to achieve a desired process result.

• The primary objective of the mixing is to achieve a homogeneous mixture, generally this means, attaining a nearly uniform distribution of the ingredients.

• The in-homogeneity can be one of concentration, phase, or temperature. Secondary effects, such as mass transfer, reaction, and product properties are usually the critical objectives.

A horse-driven mixer is a pug mill preparing clay for brick making……………

MIXING TANK

Agitated mixer are increasingly used to perform a variety of mixing tasks in

chemical products Food Biochemical Pharmaceutical Medicine Energy, Environment protection, dealing with fining, homogenizing, dissolution, gas dispersion, solid suspension, heat transfer and diffusive transport of multiple raw materials.

GEOMETRY OF MIXING TANK

A conventional stirred tank consists of a vessel equipped with a rotating mixer.

The vessel is generally a vertical cylindrical tank.

Nonstandard vessels such as those with square or rectangular cross-section, or horizontal cylinder vessels are sometimes used.

The rotating mixer has several components: an impeller, shaft, shaft seal, gearbox, and a motor drive.

Wall baffles are generally installed for transitional and turbulent mixing to prevent solid body rotation (sometimes called fluid swirl) and cause axial mixing between the top and bottom of the tank. .

Schematic of a mixing tank

MIXING MECHANISMS

Dispersion or diffusion is the act of spreading out.

Molecular diffusion is diffusion caused by relative molecular motion and is characterized by the molecular diffusivity.

Eddy diffusion or turbulent diffusion is dispersion in turbulent flows caused by the motions of large groups of molecules called eddies; this motion is measured as the turbulent velocity fluctuations.

Convection (or bulk diffusion) is dispersion caused by bulk motion.

Taylor dispersion is a special case of convection, where the dispersion is caused by a mean velocity gradient. It is most often referred to in the case of laminar pipe flow, where axial dispersion arises due to the parabolic velocity gradient in the pipe.

MEASURES OF MIXEDNESS

Scale of segregation is a measure of the large scale breakup process (bulk and eddy diffusivity) without the action of diffusion. It is the size of the packets of B that can be distinguished from the surrounding fluid A.

Intensity of segregation is a measure of the difference in concentration between the purest concentration of B and the purest concentration of A in the surrounding fluid. Molecular diffusion is needed to reduce the intensity of segregation, as even the smallest turbulent eddies have a very large diameter relative to the size of a molecule.

RESIDENCE TIME DISTRIBUTION

Residence time distributions represent the first generation of mixing models. The residence time distribution measures features of ideal or non ideal flows associated with the bulk flow patterns or macro mixing in a reactor or other process vessel.

In RTD analysis, a tracer is injected into the flow and the concentration of tracer in the outlet line is recorded over time. When the mixing is ideal or close to ideal and the reaction kinetics are known, the RTD can be used to obtain explicit solutions for the reactor yield .

The chief weakness of RTD analysis is that from the diagnostic perspective, an RTD study can identify whether the mixing is ideal or non ideal, but it is not able to uniquely determine the nature of the non ideality.

contd……

RESIDENCE TIME DISTRIBUTION

Residence time distributions are the first characteristic of mixing. The characteristic time scale for a residence time distribution is the mean residence time of the vessel. The characteristic length scale is the vessel diameter, or volume.

The conclusion is that improvements in CFD codes and still faster computers are needed for accurate design calculations in complex geometries. Residence time calculations will be a useful tool for their validation

PARAMETERS CONSIDERED

Type of mixing processLateral mixingAxial mixing

Type of flowLaminar FlowTurbulent Flow

Type of reactorBatch reactor

Type of mixers to be usedMechanical Agitators

Materials taken for the mixing process

KEY PROCESS VARIABLES

Residence time (τ)

Volume (V)

Temperature (T)

Pressure (P)

Three blade marine type Three blade marine type Double flight ribbon type:. Double flight ribbon type:.

A high efficiency turbulent flow impeller used on our smallest turbine agitators at direct drive motor speeds.

The high solidity permits operation nearer the boiling point without cavitations.

It is the most efficient blender of all existing close clearance agitators

. Generally used for applications where viscosities are ordinarily greater than 30,000 MPa.

TYPES OF IMPELLERS

Axial impeller :Axial impeller : Straight Blade Impeller : Straight Blade Impeller :

A reasonably cost effective impeller in both turbulent and laminar flow.

Good impeller for applications where the viscosity changes over a wide range causing the flow regime to vary between turbulent and laminar flow.

A reasonably cost effective impeller for solids suspension.

A cost effective impeller for operation very near the floor of a tank for agitating the heel in solids suspension applications.

TYPES OF IMPELLERS

CASE STUDY

PROCEDURE:1. Fill the overhead tanks with NaOH and Ethyl Acetate.2. Adjust the flow rates of NaOH and Ethyl Acetate until the

flow reaches steady state.3. Switch on the stirrer.4. Add 10 ml of Glacial Acetic Acid to the reactor5. Collect the samples from outlet for every 30 seconds of time

interval.6. Take 10ml from each sample and transfer it to the conical

flask which contains 10ml HCl.7. Titrate the sample with NaOH by adding phenolphthalein

indicator, till colorless solution turns to pink.8. Note down the volume of NaOH rundown.9. Repeat the same procedure for different flow rates.

FORMULAE

QNaOH * NNaOH

CAO =

QNaOH + QETHYL ACETATE

VETHYL ACETATE *ᵨ SETHYL ACETATE =

M.W.(1+VETHYL ACETATE )

CBO = QETHYL ACETATE *SETHYL ACETATE QNaOH + QETHYL ACETATE

M = CBO

CAO

CA = GNaOH

VSAMPLE

XA = 1 - CA

CAO

τ = V QNaOH + QETHYL ACETATE

OBSERVATIONS AND CALCULATIONS

EFFECT OF MIXING WITHOUT STIRRER

S.NO QNaOH

(LPH)QETHYL ACETATE

(LPH)V NaOH

RUNDOWNml

1 12.5 15 6.5

2 10 12.5 4.7

3 7.5 10 3.3

4 5 7.5 3.0

5 2.5 5 2.0

S NO τsec

XA

1 0.03709 0.0001

2 0.0453 0.010

3 0.0582 0.0109

4 0.0816 0.112

5 0.136 0.119

RESIDENCE TIME Vs CONVERSION

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

conv

ersi

on X

A

Residence time, τ (sec)

CONVERSION BY VARYING RPMs

S.No XA AT 400 RPM XA AT 600 RPM XA AT 1000 RPM τsec

1 0.0512 0.0841 0.112 0.03709

2 0.0740 0.099 0.344 0.045

3 0.0911 0.156 0.499 0.058

4 0.202 0.331 0.546 0.0816

5 0.335 0.584 0.844 0.136

RESIDENCE TIME Vs CONVERSION

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.025 0.05 0.075 0.1 0.125 0.15

conv

ersi

on X

A

Residence time, τ (sec)

XA @ 400 RPM

XA @ 600 RPM

XA @ 1000 RPM

CONVERSION WITH A THREE BLADE MARINE TYPE IMPELLER

S.No QNaOH

(LPH)

QETHYL ACETATE

(LPH)

Volume of NaOH rundown

ml

1 12.5 15 6.9

2 10 12.5 6.5

3 7.5 10 6.3

4 5 7.5 6.1

5 2.5 5 6.0

S.No XA τSec

1 O.O114

0.037

2 0.02021 0.045

3 0.02117 0.058

4 0.19905 0.0816

5 0.20704 0.136

RESIDENCE TIME Vs CONVERSION

0

0.05

0.1

0.15

0.2

0.25

0 0.025 0.05 0.075 0.1 0.125 0.15

conv

ersi

on X

A

Residence time, τ (sec)

CONVERSION WITH A STRAIGHT BLADE TYPE IMPELLER

S NO QNaOH

(LPH)QETHYL ACETATE

(LPH)V NaOH RUNDOWN

ml

1 12.5 15 7.5

2 10 12.5 6.9

3 7.5 10 6.5

4 5 7.5 6.3

5 2.5 5 6.1

S NO XA τSec

1 0.0321 0.0370

2 0.05705 0.045

3 0.06774 0.058

4 0.18905 0.0816

5 0.33903 0.136

RESIDENCE TIME Vs CONVERSION

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

cnve

rsio

n X A

Residence time, τ (sec)

CONVERSION WITH AN AXIAL HIGH EFFICIENCY IMPELLER

S NO QNaOH

(LPH)QETHYL ACETATE

(LPH)V NaOH RUNDOWN

ml

1 12.5 15 9.0

2 10 12.5 8.5

3 7.5 10 7.0

4 5 7.5 6.9

5 2.5 5 6.5

S NO XA τSec

1 0.045 0.037

2 0.101 0.045

3 0.194 0.058

4 0.310 0.0816

5 0.381 0.136

RESIDENCE TIME Vs CONVERSION

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 0.025 0.05 0.075 0.1 0.125 0.15

conv

ersi

on X

A

Residence time, τ (sec)

CONVERSION WITH A DOUBLE FLIGHT RIBBON IMPELLER

S NO QNaOH

(LPH)QETHYL ACETATE

(LPH)VOLUME OF NaOH

RUNDOWNml

1 12.5 15 8.4

2 10 12.5 7.5

3 7.5 10 6.7

4 5 7.5 6.5

5 2.5 5 6.4

S NO XA τSec

1 0.0421 0.037

2 0.0631 0.045

3 0.082 0.058

4 0.210 0.0816

5 0.348 0.136

RESIDENCE TIME Vs CONVERSION

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

conv

ersi

on X

A

Residence time, τ (sec)

COMPARISION OF VARIOUS TYPES OF IMPELLERS BY TAKING CONVERSION AS FACTOR

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16

conv

ersi

on X

A

Residence time, τ (sec)

with out impeller

three blade marine type impeller

flat 4-blade type impeller

double flight ribbon impeller

axial impeller

APPLICATIONS

Stirred tank reactors are frequently used in the chemical and biochemical industry to accomplish mixing tasks.

Stirred tank reactors are used for the mixing of various types of polymerizations, precipitations and fermentations.

A better designed and controlled mixing process leads to significant pollution prevention, better usage of raw materials and avoids expensive separation costs downstream in the process.

CONCLUSION

• From our project we were able to study the following:– Inefficient mixing has large negative effects on the yield

and selectivity of a broad range of chemical reactions, because slow mixing can retard desired reactions.

– The speed of the agitators and its involvement in the effect of mixing using a Tachometer and a Dimmerstat.

– We have taken different stirrers and achieved maximum conversion and studied the effect of mixing varying RPM and found out the properties of different impellers and their rate of mixing using different liquids.

– The best conversion we have achieved for axial impeller because of the twisted blade structure when compared with other three impellers.

SCOPE FOR FUTURE WORK

This study can be extended by varying different reactors , agitators and solutions

The study can be done in closed type vessels where different fluids can be taken.

REFERENCES

Schmidt, Lanny, The Engineering Of Chemical Reactions. NY Oxford Press, 1998.

Octave Levenspiel, The Chemical Omnibook,Oregon St Univ Bookstores 1993.

Effect Of Mixing in a Stirred Tank Reactor- Chemical Engineering Journal.

Warren L.McCabe, Julian Smith, Peter Harriot. Unit Operations Of Chemical Engineering-2005.

Bakker R A, “Micro mixing in Chemical Reactors” Thesis ,Delft University,1996.

THANK YOU