Exp 1 - Cstr Dynamic

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SOLTEQCSTR DYNAMICS (Model: BP 107)

1

CCOONNTTIINNUUOOUUSSSSTTIIRRRREEDDTTAANNKK

RREEAACCTTOORR((CCSSTTRR))DDYYNNAAMMIICC


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1.0 INTRODUCTION

In the majority of industrial chemical processes, a reactor is the key item of equipment inwhich raw materials undergo a chemical change to form desired products. The design andoperation of chemical reactors is thus crucial to the whole success of the industrial

operation. Reactors can take a widely varying form, depending on the nature of the feedmaterials and the products. Understanding non-steady behaviour of process equipment isnecessary for the design and operation of automatic control systems. One particular type ofprocess equipment is the continuous stirred tank reactor. In this reactor, it is important todetermine the system response to a change in concentration. This response ofconcentration versus time is an indication of the ideality of the system.

The SOLTLEQ CSTR Dynamics (Model BP 107) has been designed to follow thedynamics of the simplest classic case of a well-mixed, multi-staged process operation. Theunit comes with three stirred tank reactors connected in series complete with sump tanksand circulation pumps. Instruments are provided for the measurement of conductivity in

each reactor. Students may select either step change input or impulse input to the reactorand will continuously monitor the responses in each reactor at a suitable interval.


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2.0 GENERAL DESCRIPTION

Before operating the unit, students must familiarize themselves with the unit. Please referto Figure 1 to understand the process. The unit consists of the followings:

a) ReactorsThree reactors made of borosilicate glass, each having approximately 2 literscapacity. Each reactor is fitted with variable speed stirrer mounted on the top plate.Temperature and conductivity sensors are provided for each reactor. Flowsbetween vessels are by gravity. Overflow tubes are provided for the 2nd and 3rdreactor.

b) Stirrer SystemVariable speed stirrer system with digital display consisting of a motor and a shaftwith impellers made of stainless steel. Speed adjustments by means of a speedcontroller knob on each stirrer.

c) Feed TanksTwo 15-L cylindrical tanks made of stainless steels are provided with the unit. Eachtank has a feed pump to transfer the liquid from feed tank to the reactors. Eachtank is fitted with a level switch to protect the pumps from dry run.

d) Waste TankA rectangular 50-L waste tank made of stainless steel is provided at the bottom ofthe equipment.

e) Dead Time CoilMaterial : 3/8 stainless steel tubingVolume : approx. 200 ml

f) Instrumentations

Flowmeter:Range : 0 to 500 ml/minOutput : 0 to 5 VDCDisplay : LCD digital display

Conductivity Meter:Sensor Range : 0 to 200 mS/cmNo. of Sensors : 4 (CT1, CT2, CT3, CT4)Output : 4 to 20 mADisplay : conductivity controller with digital display for each sensor

mounted on the control panel

Temperature Sensor:No. of Sensors : 3 (TT1, TT2, TT3)Sensor type : RTD


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g) Data Acquisition SystemThe Data Acquisition System consists of a personal computer, ADC modules andinstrumentations for measuring the process parameters. A flowmeter with 0 to 5VDC output signal is supplied for feed flowrate measurement. Conductivity sensorswith controller are provided for monitoring the tracer concentration in each reactor.

All analog signals from the sensors will be converted by the ADC modules intodigital signals before being sent to the personal computer for display andmanipulation.


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Figure 1. Process Diagram for CSTR Dynamics (BP 107).


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3.0 SUMMARY OF THEORY

Consider a stirred tank as of Figure 1. The fundamental quantities whose values providethe information about the dynamics of the system are:

a.

The total mass of the liquid in the tankb. The total energy of the material in the tankc. Its momentum

Figure 2. Stirred Tank.

Type of Reactor Characteristics

Continuously Stirred TankReactor (CSTR)

Run at steady state with continuous flow of reactants and products;the feed assumes a uniform composition throughout the reactor, exitstream has the same composition as in the tank

Kinds of PhasesPresent

Usage Advantages Disadvantages

1. Liquid phase

2. Gas-liquidrxns

3. Solid-liquidrxns

1. Whenagitation isrequired

2. Seriesconfigurationsfor differentconcentrationstreams

1. Continuous operation

2. Good temperature control

3. Easily adapts to two phaseruns

4. Good control

5. Simplicity of construction

6. Low operating (labor) cost

7. Easy to clean

1. Lowestconversion perunit volume

2. By-passing andchannelingpossible withpoor agitation


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General Mole Balance Equation

dt

dNdVrFF

A

V

AAA

0

0

Figure 3.

Assumptions:

1. For steady state 0dtdNA

.

2. Well mixed, therefore rAis the same throughout the reactor.

VrdVrdVrA

V

A

V

A

00

Rearranging the generation,

A

AA

r

FFV

0

In terms of conversion,

A

A

A

AA

r

XFV

F

FFX

0

0

0

Reactors in SeriesGivenrAas a function of conversion, rA= f(X), one can also design any sequence ofreactors in series provided there are no side streams by defining the overall conversion atany point.


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reactorfirsttofedAofmoles

pointtoupreactedAofmoles iX

i

Figure 4. Reactors in Series

Mole Balance on Reactor 1:

0

0GenerationOutIn

1110

VrFFAAA

1

101

1001

0

101

A

A

AAA

A

AA

r

XFV

XFFF

F

FFX

Mole Balance on Reactor 2:

0

0GenerationOutIn

2221

VrFFAAA

2

1202

2002

0

202

)(

A

A

AAA

A

AA

r

XXFV

XFFF

F

FFX


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GivenrA= f(X),the Levenspiel Plot can be used to find the reactor volume.

Figure 5. Reactor Volume Plot.

Example of Design Equation for Reactor in Series:

Acetic acid is hydrolysed in three stirred tank reactors operated in series. The feedflows to the first reactor (V = 1 lit) at a rate of 400 cm 3/min. The second and thirdreactors have volumes of 2 and 1.5 litres respectively. The first order irreversiblerate constant is 0.158 min-1. Calculate the fraction hydrolysed in the effluent from the

third reactor.

Calculations:

The design equation for series, steady flow mixed reactor isVi=FAo(XA,i- XA,i-1) / (-rA)iWhere Vi= volume of reactor iFAo= molal flow rate of A into the first reactorXA,i= fractional conversion of A in the reactor iXA,i-1= fractional conversion of A in the reactor i-1For first order reaction, -rA,i= kCA,i= kCAo(1 - XA,i)

v = volumetric flow rate of A = 400 cm3/min = 0.4 lit/min

For the first reactor: (V = 1 lit)(-rA)1= (kCA)1= k CA,1= k CAo( 1- XA,1)CAo= FAo/ vi.e., FAo= v CAoXA,i-1= XA,0= 0Therefore,Vi=FAo(XA,i- XA,i-1) / (-rA)i1 = 0.4 (XA,1- 0) / (0.158 x ( 1 - XA,1 ) )XA,1= 0.283


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For the second reactor: (V = 2 lit)(-rA)2= (kCA)2= k CA,2= k CAo( 1- XA,2)

Therefore,(-rA)2= k CAo( 1- XA,2)

XA,1= 0.283FAo= v CAoVi=FAo(XA,i- XA,i-1) / (-rA)i2 = 0.4 (XA,2- 0.283) / ( k ( 1- XA,2) )5 k = (XA,2- 0.283) / ( 1- XA,2)0.79 - 0.79 XA,2= XA,2- 0.2831.073 = 1.79 XA,2XA,2= 0.60

For the third reactor: (V = 1.5 lit)(-rA)3= (kCA)3= k CA,3= k CAo( 1- XA,3)

XA,2= 0.6FAo= v CAoVi=FAo(XA,i- XA,i-1) / (-rA)i1.5 = 0.4 (XA,3- 0.60) / ( k ( 1- XA,3) )0.5925 = (XA,3- 0.60) / ( 1- XA,3)0.5925 - 0.5925 XA,3= XA,3- 0.601.1925 = 1.5925 XA,3XA,3 = 0.749The fraction hydrolyzed in the effluent from the third reactor = 0.749


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Tracer Analysis on the Transient Behaviour of Continuous Stirred-Tank in Series

Unlike the above, the tracer analysis will help us understand the transient behaviour of thecontinuous stirred tank reactor in series by having a step input or pulse of tracercomponent such as salts. The conductivity measurement will indicate the progression of

the tracer throughout the stirred tank in series.

Figure 6.

)( 11 ii CCdtdC where vV and V=Tank Volume, v = volume flow rate, and Ci= concentration in ith Tank. The differential equations must be solved simultaneously.

A real reactor will be modeled as a number of equally sized tanks-in-series. Each tankbehaves as an ideal CSTR. The number of tanks necessary, n (our one parameter), is

determined from the E(t) curve.

Figure 7.


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For n tanks in series, E(t) is,

n

i

tn

n

ettE

i

/)1()(

1

where, ni

It can be shown that

im nt

In dimensionless form

i

i

tn

ntt

0

2

2

22

2

0

2

2

22

1

)()1(

)()(

)1(

)()()(

dE

dtEt

n

enntEE

nn

Carrying out the integration for the n tanks-in-series E(t),

2

2

2

22 1

n

n

For a first order reaction,

n

ikX )1(

1

1 , ni

For reactions other than first order and for multiple reactions, the sequentialequations must be solved.


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An

AnnA

i

A

AA

i

A

AA

i

i

r

CCvV

r

CC

vV

r

CCvV

nVV

)(

)(

)(

)1(

0

2

210

1

100

Example:

For a second order reaction with n = 3,

(V1= V2= V3= V/3)

02

03

)(v

kC

CCV

A

AA

)( 321

k

kC

C

CCkC

AA

AAA

3

031

2

3

2

411

0

Similarly,

3,1

2

411

2

411

3

0

3

3

23

3

3

13

2

A

A

A

A

A

A

C

CX

k

kCC

k

kCC


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Effect of Step Change in Input Concentration to the Concentration of Solute inStirred Tank Reactors in Series.

When a step change of solute concentration is introduced at the feed of tank 1, the tank inseries will experience a transient behaviour as of Figure 8 below. The response will be

dependent on the residence time of each reactor in series.

Figure 8a. Step change input. Figure 8b. Transient response of tank in seriesto the step input.

Effect of Pulse in Input Concentration to the Concentration of solute in Stirred Tankin Series.

When a pulse input of solute concentration is introduced at the feed of tank 1, the transientbehaviour will be different than the step change input due to the diminishing concentrationfrom the input after pulsing as described in Figure 9.

Figure 9a. Pulse input. Figure 9b. Transient response of tank in seriesto the pulse input.


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References

Levenspiel, O., Chemical Reaction Engineering, John Wiley, 1972.Fogler, H.S., Elements of Chemical Reaction Engineering, 3rdEdition, Prentice Hall PTR, 1999.Smith, J.M., Chemical Engineering Kinetics, McGraw Hill, 1981.

Astarita, G., Mass Transfer with Chemical Reaction, Elsevier, 1967.


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4. EXPERIMENTAL PROCEDURES

4.1 Experiment A: The Effect of Step Change Input

In this experiment a step-change input would be introduced and the progression of

the tracer will be monitored via the conductivity measurements in all the threereactors and after the dead time coil.

1. Fill up the feed tank T1 with deionised water.

2. Prepare about 10-L of 0.025M sodium chloride solution in feed tank T2.

3. Set the 3-way valve V3 position towards pump P1. Close valve V6.

4. Open valve V5. Switch on Pump P1 to initially fill up all three reactors withdeionised water. Record each reactor volume.

5. Adjust needle valve V4 to obtain a flowrate of approximately 150 ml/min

on flowmeter FT1. Make sure that no air bubbles are trapped in the piping.Note: It is important to maintain the liquid level in each reactor. Adjust

the flowrate if necessary.

6. Switch on stirrers 1, 2 and 3. Set the stirrer speed to approximately 200rpm.

7. Continue pumping the de-ionized water until all conductivity readings(CT1, CT2, CT3) are stable at low values.

8. Record these conductivity values at time t0.

9. Switch off pump P1. Quickly set the 3-way valve V3 position towards

pump P2. Switch on pump P2 and start the timer simultaneously.

10. Record all conductivity values (CT1, CT2, CT3) at a suitable interval in anappropriate table.

11. Continue recording the conductivity values until all readings are almostconstant.

12. Switch off pump P2. Close valve V4.

13. Drain all liquids in each reactor by opening valves V11, V12, and V13.

Note:

For operations with SOLDAS Data Acquisition System, refer to the DAS operatingprocedure. In step 9, click the START button. Conductivity values will be recordedautomatically and a table will be generated.


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4.2 Experiment B: The Effect of Pulse Input

In this experiment a pulse input would be introduced and the progression of thetracer will be monitored via the conductivity measurements in all the three reactorsand after the dead time coil.

1. Fill up feed tank T1 with deionised water.


3. Set the three-way valve V3 position towards pump P1. Close valve V6.

4. Open valve V5. Switch on Pump P1 to fill up all three reactors withdeionised water. Record each reactor volume.

5. Adjust needle valve V4 to obtain a flowrate of approximately 150 ml/minon flowmeter FT1. Make sure that no air bubbles are trapped in the piping.

Note: It is important to maintain the liquid level in each reactor. Adjustthe flowrate if necessary.

6. Switch on stirrers 1, 2 and 3. Set the stirrer speed to approximately 200rpm.

7. Continue pumping the deionised water until all conductivity readings (CT1,CT2, CT3) are stable at low values.

8. Record these conductivity values at time t0.

9. Switch off pump P1. Quickly set the 3-way valve V3 position towardspump P2. Switch on pump P2 and start the timer simultaneously.

10.

Let pump P2 to operate for 2 minutes, and then switch off pump P2.Quickly switch the 3-way valve V3 position back towards pump P1. Switchon pump P1 and let it run till the end of experiment.

11. Record all conductivity values (CT1, CT2, CT3) at a suitable interval in anappropriate table.

12. Continue recording the conductivity values until all readings are almostconstant.

13. Switch off pump P1. Close valve V4.

14. Drain all liquids in reactors by opening valves V11, V12 and V13.

Note:



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4.3 Experiment C: The Investigation on Dead Time Coil

In this experiment the modeling of real (non-ideal) reactor can be accomplished byassembling the CSTR and dead time coil in parallel. A pulse input would beintroduced in such a way that a fraction of it passes through the coil. This is

analogous to the existing of a stagnation zone in a real reactor. Then, theprogression of the tracer will be monitored via the conductivity measurements atthe exit.

1. Fill up feed tank T1 with de-ionized water.


3. Set the three-way valve V3 position towards pump P1.

4. Open valves V6 and V10. Close valves V5 and V15. Switch on Pump P1and allow the liquid to flow slowly through the coil for few minutes to bleedoff any air trapped.

5. Close valve V10. Open valve V15.

6. Adjust needle valve V4 to obtain a flowrate of approximately 150 ml/minon flowmeter FT1. Make sure that no air bubbles are trapped in the piping.

Note: It is important to maintain the liquid level in reactor R3. Adjust theflowrate if necessary.

7. Switch on stirrer 3. Set the stirrer speed to approximately 200 rpm.

8. Continue pumping the de-ionized water until conductivity reading (CT3) isstable at low values.

9.

Record this conductivity value at time t0.

10. Switch off pump P1. Quickly set the 3-way valve V3 position towardspump P2. Switch on pump P2 and start the timer simultaneously.

11. Let pump P2 to operate for 2 minutes, and then switch off pump P2.Quickly switch the 3-way valve V3 position back towards pump P1. Switchon pump P1 and let it run till the end of experiment.

12. Record conductivity value (CT3) at a suitable interval (e.g. 30 sec) in anappropriate table.

13. Continue recording the conductivity value until readings are almost

constant.

14. Repeat the experiment (steps 5 to 13) but this time allow the liquid topartially flow through the dead time coil. This is done by adjusting valveV15 to get the right flow while valve V10 remains open.

15. Stop the experiment and drain all liquids in reactors by opening valvesV11, V12 and V13.

Note:



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5. MAINTENANCE

1. After each experiment, drain off any liquids from the reactor and make sure thatthe reactor and tubings are cleaned properly. Flush the system with de-ionizedwater until no traces of salt are detected.

2. Dispose all liquids immediately after each experiment. Do not leave any solution orwaste in the tanks over a long period of time.

3. Wipe off any spillage from the unit immediately.

6. SAFETY PRECAUTIONS

1. Always observe all safety precautions in laboratory.

2. Always wear protective clothing, shoes, helmet and goggles throughout thelaboratory session.

3. Always run the experiment after fully understand the equipment and procedures.

4. Always plug in all cables into appropriate sockets before switching on the mainpower on the control panel. Inspect all cables for any damage to avoid electricalshock. Replace if necessary.

5. Make sure that the stirrer assembly is secured properly. Avoid excessive load tothe electrical motor to prevent damage.

6. Inspect the unit, including tubings and fittings, periodically for leakage and wornout. Leakage might cause damage to equipments by corrosive chemical in the longrun.


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APPENDIX

Exp 1 - Cstr Dynamic

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