ABSTRACT

From this experiment, our objectives are to examine the effect of pulse input and step change

in a tubular flow reactor and to construct a residence time distribution (RTD) function for the

tubular flow reactor. In the pulse input experiment, the flow rate was set up at 700 mL/min

and let it for one minute before the valve is open and the reading taken every 30 seconds until

the conductivity reading is 0.0. In the other hand, the step change experiment, the

conductivity were observe every 30 seconds until the reading at outlet conductivity is

constant for 3 times in which in this experiment the constant value was 2.6. For the first

experiment, the final conductivity for inlet and outlet are 0.0 mS/cm and 0.0 mS/cm while for

the second experiment are 3.9 mS/cm and 2.6 mS/cm. The outlet conductivity, C(t) is

calculated and we get 5.3 for the first experiment and 8.3 for the second experiment. Then,

we are able to determine the distribution of exit time, E(t) for each 30 seconds. The sum of

E(t) we get is 1.0000 for both experiment which is the residence time distribution for the

experiment. The total mean residence time, tm for both experiment are 0.2465 minute and

0.4354 minutes respectively. The sum of variance, σ2 and the skewness, s3 are also then

calculated. The value we get for both experiment for σ2 is 0.3665 and 1.6245, for the s3 is

0.6992 and 6.5121 respectively. Graphs for outlet conductivity, C(t) against time and

distribution of exit time, E(t) against time is plotted. The plotted graph shows that the value

of E(t) is depends on the value of C(t) in which is the same concept with the theory.

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TABLE OF CONTENT

PAGE

ABSTRACT 1

TABLE OF CONTENT 2

INTRODUCTION 3

OBJECTIVE 4

THEORY 4

APPARATUS 5

PROCEDURE 6

RESULTS 7

SAMPLE OF CALCULATION 12

DISCUSSION 13

CONCLUSION 14

RECOMMENDATION 14

REFERENCES 15

APPENDIXES 16

2

INTRODUCTION

A tubular reactor is a vessel through which flow is continuous, usually at steady state,

and configured so that conversion of the chemicals and other dependent variables are

functions of position within the reactor rather than of time. In the ideal tubular reactor, the

fluids flow as if they were solid plugs or pistons, and reaction time is the same for all flowing

material at any given tube cross section. Tubular reactors resemble batch reactors in

providing initially high driving forces, which diminish as the reactions progress down the

tubes. Flow in tubular reactor can be laminar or turbulent. Turbulent flow generally is

preferred to laminar flow, because mixing and heat transfer are improved.

The reactants are continually consumed as they flow down the length of the reactor in

the tubular reactor. However, many tubular reactors that are used to carry out a reaction do

not fully conform to this idealized flow concept. In an ideal plug flow reactor, a pulse of

tracer injected at the inlet would not undergo any dispersion as it passed through the reactor

and would appear as a pulse at the outlet. The degree of dispersion that occurs in a real

reactor can be assessed by following the concentration of tracer versus time at the exit. This

procedure is called the stimulus-response technique.

For most chemical reactions, it is impossible for the reaction to proceed to 100%

completion. The rate of reaction decreases as the per cent completion increases until the point

where the system reaches dynamic equilibrium (no net reaction, or change in chemical

species occurs). The equilibrium point for most systems is less than 100% complete. For this

reason a separation process, such as distillation, often follows a chemical reactor in order to

separate any remaining reagents or by products from the desired product. These reagents may

sometimes be reused at the beginning of the process, such as in the Haber process.

High temperature reactions Residence Time Distribution (RTD) analysis is a very

efficient diagnosis tool that can be used to inspect the malfunction of chemical reactors. It can

also be very useful in the estimation of effluent properties and in modelling reactor

behaviour. This technique is extremely important in teaching reaction engineering, in

particular when the non-ideal reactors become the issue. Residence time distributions are

measured by introducing an impulse and step tracer technique into the system. The

concentration of the tracer is changed according to a known function and the response is

3

found by measuring the concentration of the tracer at the outlet. The RTD technique has also

been used for the experimental characterization of flow pattern of a packed bed and a tubular

reactor that exhibit, respectively, axially dispersed plug flow and laminar flow patterns.

Another important field of RTD applications lies in the prediction of the real reactor

performance. Nowadays, the concepts of macro and micro mixing are fundamental. Each

macro mixing level is expressed in the form of a specific RTD. There is a given micro mixing

level, which lies between two limiting cases, complete segregation and perfect micro mixing.

OBJECTIVE

1) To examine the effect of pulse input in tubular flow reactor.

2) To examine the effect of a step change input in a tubular flow reactor.

3) To construct a residence time distribution (RTD) function for the tubular flow reactor.

THEORY

Tubular reactors are specially designed to allow detailed study of important process.

The tubular reactor is one of three reactor types which are interchangeable on the reactor

service unit. The reactions are monitored by conductivity probe as the conductivity of the

solution changes with conversion of the reactant to product. This means that the inaccurate

and inconvenient process of titration, which was formally used to monitor the reaction

progress, is no longer necessary.

In a tubular flow reactor, the feed enters at one end of a cylindrical tube and the

product stream leaves at the other end. The long tube and the lack of provision for stirring

prevent complete mixing of the fluid in the tube. Hence the properties of the flowing stream

will vary from one point to another, namely in both radial and axial directions. It is often not

necessary to know details of the entire flow fluid but rather only how long fluid elements

reside in the reactor (i.e. the distribution of residence times). This information can be used as

a diagnostic tool to ascertain flow characteristics of a particular reactor.

4

Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter

tubes, and greatly deviate from ideal plug-flow behaviour, or turbulent, as with gases.

Turbulent flow generally is preferred to laminar flow, because mixing and heat transfer are

improved. For slow reactions and especially in small laboratory and pilot-plant reactors,

establishing turbulent flow can result in inconveniently long reactors or may require

unacceptably high feed rates.

In order to analyse the residence time distribution of the fluid in a reactor the

following relationships have been developed. Fluid elements may require differing lengths of

time to travel through the reactor. The distribution of the exit times, defined as the E(t) curve,

is the RTD of the fluid. The outlet conductivity of a tracer species C(t) can be used to define

E(t). That is:

E (t )≈Coutt (t)

∫0

∞

Coutt ( t )dt

Based on the data collected, a graph of conductivity versus time could be draw to

obtain the C(t) curve and data of the integral C(t) could be calculate.

∫0

∞

C ( t )dt=∑Ci∆ t=Area

If the RTD function, E(t), is very broad, however, it may be difficult to inject an

amount of tracer that is sufficiently large so as to keep the outlet concentration sufficiently

high to be measured accurately.

APPARATUS

1) Soltec Tubular Flow Reactor instrument

2) Clock watch

3) Solution 0.025M Sodium Chloride and De-ionized water.

5

PROCEDURE

Experiment 1:

1. Perform the general start-up procedure as the manual instructed.

2. Valve V9 is opened and pump P1 is switched on.

3. The P1 pump is adjusted by controlling the flow controller to obtain a flow rate of

approximately 700mL/min at Fl-01 of de-ionized water into the reactor R1.

4. The de-ionized water is allowed to continue flowing through the reactor until the inlet

(Q1-01) and outlet (Q1-02) conductivity values are stable at low levels. Both

conductivity values are recorded.

5. Valve V9 is closed and pump P1 is switched off.

6. Valve V11 is opened and pump P2 is switched on. The timer is simultaneously

started.

7. Pump P2 flow controller is adjusted to give a constant flow rate of salt solution into

the reactor R1 at 700mL/min at F1-02.

8. The salt solution is allowed to flow for 1 minute, the timer is reset and restarted. This

will start the time at the average pulse input.

9. Valve V11 is closed and pump P2 is switched off. Valve V9 is quickly opened and

pump P1 is switch on.

10. By adjusting pump P1 flow controller, the de-ionized water flow rate is always

maintained at 700mL/min.

11. The inlet (Q1-01) and outlet (Q1-02) conductivity values are recorded at regular

interval of 30 seconds.

12. The conductivity values are recorded until all readings are almost constant and

approach stable low level values.

Experiment 2:

1. Closed all the pump and valve from the experiment 1.

2. Valve V9 is opened and pump P1 is switched on.

3. The P1 pump is adjusted by controlling the flow controller to obtain a flow rate of

approximately 700mL/min at Fl-01 of de-ionized water into the reactor R1.

6

4. The de-ionized water is allowed to continue flowing through the reactor until the inlet

(Q1-01) and outlet (Q1-02) conductivity values are stable at low levels. Both

conductivity values are recorded.

5. Valve V9 is closed and pump P1 is switched off.

6. Valve V11 is opened and pump P2 is switched on. The timer is simultaneously

started.

7. The inlet (Q1-01) and outlet (Q1-02) conductivity values are recorded at regular

interval of 30 seconds.

8. The conductivity values are recorded until all readings are almost constant.

RESULTS

Experiment 1: Pulse input in a Tubular Flow Reactor

Flow rate = 700 mL/min

Input type = Pulse input

Time (min)

Conductivity (ms/cm)

Inlet (Co) Outlet (Ci)

0.0 0.2 2.6

0.5 0.2 2.6

1.0 0.1 2.6

1.5 0.0 2.7

2.0 0.0 2.0

2.5 0.0 0.5

3.0 0.0 0.1

3.5 0.0 0.1

4.0 0.0 0.0

4.5 0.0 0.0

5.0 0.0 0.0

7

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

Outlet Conductivity vs Time

Time (min)

Out

let C

ondu

ctivi

ty (m

S/cm

)

Time (min) Conductivity

Outlet (Ci)

C(t)

Ci∆t

E(t)

Ci(∆t)

∑Ci(∆t)

0.0 2.6 0.0000 0.0000

0.5 2.6 1.3000 0.2453

1.0 2.6 1.3000 0.2453

1.5 2.7 1.3500 0.2547

2.0 2.0 1.0000 0.1887

2.5 0.5 0.2500 0.0472

3.0 0.1 0.0500 0.0094

3.5 0.1 0.0500 0.0094

4.0 0.0 0.0000 0.0000

4.5 0.0 0.0000 0.0000

5.0 0.0 0.0000 0.0000

Residence time distribution (RTD) function for tubular flow reactor

8

0 1 2 3 4 5 60

0.05

0.1

0.15

0.2

0.25

0.3

E(t) vs Time

Time (min)

E(t)

Time (min) Outlet

(Ci)

E(t) tm= tE(t) σ2 =

(t - tm)2* E(t)/

∑C i∆ t

S3 =

(t - tm)3* E(t)/

∑C i∆ t

0.0 2.6 0.0000 0.0000 0.0000 0.0000

0.5 2.6 0.2453 0.0231 0.0105 0.0050

1.0 2.6 0.2453 0.0463 0.0421 0.0401

1.5 2.7 0.2547 0.0721 0.0980 0.1399

2.0 2.0 0.1887 0.0712 0.1325 0.2555

2.5 0.5 0.0472 0.0223 0.0547 0.1355

3.0 0.1 0.0094 0.0053 0.0071 0.0476

3.5 0.1 0.0094 0.0062 0.0216 0.0756

4.0 0.0 0.0000 0.0000 0.0000 0.0000

4.5 0.0 0.0000 0.0000 0.0000 0.0000

5.0 0.0 0.0000 0.0000 0.0000 0.0000

Total (∑) = 1.0000 0.2465 0.3665 0.6992

Experiment 2: Step change input in a Tubular Flow Reactor

9

Flow rate = 700 mL/min

Input type = Step change

Time (min)

Conductivity (ms/cm)

Inlet (Co) Outlet (Ci)

0.0 0.0 0.0

0.5 3.6 0.0

1.0 3.7 0.0

1.5 3.8 0.0

2.0 3.8 1.6

2.5 3.9 2.3

3.0 3.9 2.4

3.5 3.9 2.5

4.0 3.9 2.6

4.5 3.9 2.6

5.0 3.9 2.6

0 1 2 3 4 5 60

0.5

1

1.5

2

2.5

3

Outlet conductivity vs Time

Time (min)

Oul

et C

nduc

tivity

(mS/

cm)

Time

(min)

Conductivity (mS/cm) C(t) E(t) tm σ2 S3

Inlet Outlet Ci∆t Ci(∆t) t*E(t)/ (t - tm) 2 * (t - tm) 3 *

10

(Co) (Ci) ∑Ci(∆t) ∑C i∆ t E(t)/∑C i∆ t

E(t)/∑C i∆ t

0.0 0.0 0.0 0.0000 0.0000 0.0000 0.0000 0.0000

0.5 3.6 0.0 0.0000 0.0000 0.0000 0.0000 0.0000

1.0 3.7 0.0 0.0000 0.0000 0.0000 0.0000 0.0000

1.5 3.8 0.0 0.0000 0.0000 0.0000 0.0000 0.0000

2.0 3.8 1.6 0.8000 0.0964 0.0232 0.0454 0.0897

2.5 3.9 2.3 1.1500 0.1386 0.0417 0.1009 0.2481

3.0 3.9 2.4 1.2000 0.1446 0.0523 0.1514 0.4462

3.5 3.9 2.5 1.2500 0.1506 0.0635 0.2143 0.7364

4.0 3.9 2.6 1.3000 0.1566 0.0755 0.2906 1.1404

4.5 3.9 2.6 1.3000 0.1566 0.0849 0.3678 1.6238

5.0 3.9 2.6 1.3000 0.1566 0.0943 0.4541 2.2275

Total (∑) = 8.3000 1.0000 0.4354 1.6245 6.5121

Residence time distribution (RTD) function for tubular flow reactor

0 1 2 3 4 5 60

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

E(t) vs Time

Time (min)

E(t)

SAMPLE OF CALCULATION

11

∫0

∞

C (t)dt=∑C i∆ t=Area

Area = (0.0X0.0)+(0.0X0.5) +(0.0X0.5)+(0.0X0.5)+(1.6X0.5)+ (2.3X0.5)+(2.4X0.5)

+(2.5X0.5)+(2.6X0.5) +(2.6X0.5) +(2.6X0.5).

Area = 8.3000

tm=t ×E (t)Area

tm=2.0×0.0964

8.3000

tm=0.0232

σ 2=( t−tm )2E(t )Area

σ 2=(2.0−0.0232 )2×0.0964

8.3000

σ 2=0.0454

s3=(t−tm )3 E(t)Area

s3=(2.0−0.0232 )3×0.0964

8.3000

s3=0.0897

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DISCUSSION

The objectives that need to be achieve for this tubular reactor experiment is to

examine the effect of a pulse input and step change in a tubular reactor and also to construct

the residence time distribution (RTD) function for the tubular flow reactor at the end of the

experiment. De-ionized water and Salts solution was used as chemicals to determine the

conductivity. The experiment was run at flow rate of 700mL/min. The conductivity for the

inlet and outlet was recorded from time equal to t0=0 until them both reaching a constant

value for itself.

For this experiment, the graph of outlet conductivity versus times had been plotted.

Based on graph of pulse input, the outlet conductivity that had been plotted is 2.6 mS/cm

during the experiment started which is the highest value. After that, the conductivity is

decrease within the time and comes to be constant at the time of 3.5 minutes. From the result,

it showed that the results was not differ from the theory that recorded that the conductivity is

reaching zero at time of 4 minutes. Thus, the experiment 1 is succeeded. However, some of

the unexpected error may occur during the experiment because the outlet conductivity should

starts lower and increase and decrease again so that we can calculate the area under the graph

more accurate.

In addition, for the graph of step change the outlet conductivity is increases within the

time by started at time of 2.0 minutes which it inlet conductivity is 3.8 mS/cm and then

increase until at minutes 3.5 and then the value of the outlet conductivity is constant at 2.6

mS/cm till minutes 5.

To construct a residence time distribution (RTD) function for the tubular flow reactor

with pulse input and step change, the graph based on exit time (E(t)) versus time were

plotted. Both the RTD and outlet conductivity versus time graph were almost the same for

both input. From the graph, it can be concluded that the residence time distribution is depends

on the outlet conductivity.

13

CONCLUSION

From the experiment, we able to examine the effect of the pulse input and step change

in a tubular flow reactor and we also can differentiate both of the effect. Besides, we also able

to construct the residence time distribution (RTD) function for the tubular flow reactor. For

experiment 1, the effect of pulse input in a tubular flow reactor was examined. The flow rate

was kept constant at 700 ml/min. The summation of C(t) is 5.3, and the sum of E(t) came to a

result of 1.0000. As seen from the graph, both outlet conductivity and E(t) remained constant

until minute 1.5, then took a sharp decrease until minute 3, and then the outlet conductivity is

constant 0 mS/cm till minutes 5. For experiment 2, the effect of a step change input was

examined. The flow rate was also kept constant at 700 ml/min. The results and calculations

shows that the summation of the conductivity was 8.3000 and the sum of E(t) was 1.0000.

From the both graphs plotted, it is seen that the data is constant till minute 1.5 and then

increases sharply until minute 4 after that, the graph is constant. The experiment was

considered a success as all objectives were achieved.

RECOMMENDATION

There are several recommendations that can be taken in order to get more accurate result or

lower the percentage error that are:

1) Make sure that certain valve need to be open and closed rapidly, so one person must

handle this valve with efficiently to get more accurate reading.

2) The flow rate of fluid in the reactor must constant all the time during the experiment.

This is because the flow rate is always reset when we switch on and off the pump.

3) To obtain more understanding about this experiment, its recommend to use another

solvent and solute.

14

REFERENCES

Jorge A.T.A.D., Paula R.P., Jorge A.W.G., (2013). Determination of the effective radial mass

diffusivity in tubular reactors under non-Newtonian laminar flow using residence time

distribution data. International Journal of Heat and Mass Transfer. Vol. 71. 18-25.

National Technical University of Athens (NTUA). (n.d.). Tubular reactor or plug flow

reactor. http://www.metal.ntua.gr/~pkousi/e-learning/bioreactors/page_07.htm.

Reactor pilot plant manual laboratories provided by instructor.

Kanse N.G., Dawande S.D. (2012). RTD Studies in Plug Flow Reactor and its Simulation

with Comparing Non Ideal Reactors. Research Journal of Recent Sciences Vol. 1(2),

42-48.

http://caltechbook.library.caltech.edu/274/9/FundChemReaxEngCh8.pdf

15

APPENDIXES

Figure 1: Soltec Tubular Flow Reactor instrument

16