Tubular

ABSTRACT

This experiment is carried out to examine the effect of pulse input and step change in tubular flow reactor as well as to construct a residence time distribution (RTD) function for the tubular flow reactor. First of all, the equipment is set up before we run the experiment. After that, we set up the flowrate to 700mL/min. After the conductivity for inlet and outlet we collected are reaching to a constant value, the experiment is stopped. For the first experiment, the conductivity for inlet and outlet are 0.1mS/cm and 0.1mS/cm while for the second experiment are 3.2mS/cm and 2.1 mS/cm. The outlet conductivity, C(t) is calculated and we get 2.3 for the first experiment and 3.85 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 1.0035 and 0.9999 which is the residence time distribution for both of the experiment. The mean residence time, tm for this experiment are 0.8410 minute and 0.8618 minutes respectively. The variance, σ2 and the skewness, s3 are also then calculated. The value we get for σ2 is 1.3091 and for the s3 is 2.6107 for the first experiment. Meanwhile, the value of σ2 is 2.5459 and for the s3 is 8.2451 for the second experiment. Graphs for outlet conductivity, C(t) against time and distribution of exit time, E(t) against time is plotted. The graphs we get from this experiment are just the same with the graphs in the theory. The value of E(t) is depends on the value of C(t).

INTRODUCTION

The reactants are continually consumed as they flow down the length of the reactor in the tubular reactor. Flow in tubular reactor can be laminar or turbulent. 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 conveniently long reactors or may require unacceptable high feed rates.

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However, many tubular reactors that are used to carry out a reaction do not fully conform to this idealized flow concept. 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 in an ideal plug flow reactor. The degree of dispersion that occurs in a real reactor can be measured by following the concentration of tracer versus time at the exit. This procedure is known as the stimulus-response technique. The nature of the tracer peak gives an indication of the non-ideal that would be characteristic of the reactor. It is impossible for the reaction to proceed to 100% completion for most chemical reactions. The percent completion increases as the rate of reaction decreases until the point where the system reaches dynamic equilibrium. The equilibrium point for most systems is less than 100% complete. For this reason in order to separate any remaining reagents from the desired product, a separation process often follows a chemical reactor. These reagents may sometimes be reused at the beginning of the process. Residence Time Distribution (RTD) analysis can be used to inspect the malfunction of chemical reactors. It can also be very useful in the estimation of effluent properties and in modeling reactor behavior. This technique is extremely important in teaching reaction engineering, in particular when the non-ideal reactors become the issue. By impulse and step tracer injection techniques can determine RTDs, and applying them to the modeling of the reactor flow and to the estimation of the behavior of a nonlinear chemical transformation. 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 (FEUP). 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.

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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

In a tubular 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. In the ideal tubular reactor, which is called the “plug flow” reactor, specific assumptions are made about the extent of mixing:

1. no mixing in the axial direction, i.e., the direction of flow 2. complete mixing in the radial direction 3. a uniform velocity profile across the radius.

The absence of longitudinal mixing is the special characteristics of this type of reactor. It is an assumption at the opposite extreme from the complete mixing assumption of the ideal stirred tank reactor. The validity of the assumptions will depend on the geometry of the reactor and the flow conditions.

Deviations, which are frequent but not always important, are of two kinds: 1. mixing in longitudinal direction due to vortices and turbulence 2. incomplete mixing in radial direction in laminar flow conditions 3

Mass Balance For a time element ∆t and a volume element ∆V, the mass balance for species „i‟ is given by the following equation: QA CA │v ∆t- QA CA│v+∆v ∆t - rA∆V∆t = 0

(10.1.1)

Where QA : volumetric flow rate of reactant A to the reactor, L/s CA : concentration of reactant A, mol/L rA : rate of disappearance of reactant A, mol/L•s

The conversion, X, is defined as:

X = (initial concentration - final concentration) / (initial concentration)

Since the system is at steady state, the accumulation term in Equation (10.1.1) is zero.

Equation (10.1.1) can be written as: -QA ∆CA - rA∆V = 0

(10.1.2)

Dividing by ∆V and taking limit as ∆V → 0

dCA/dV = -rA/QA

(10.1.3)

This is the relationship between concentration and size of reactor for the plug flow reactor. Here rate is a variable, but varies with longitudinal position (volume in the reactor, rather than with time). 4

Integrating,

-dV/ QA = dCA/rA

At the entrance:

(10.1.4)

V=0 CA = CA0

At the exit:

V = VR (total reactor volume) CA = CA (exit conversion)

-

VR = ∫ dCA QA

rA

APPARATUS

1. SOLTEC Tubular Flow Reactor Instrument. 2. Clock watch 3. De-ionized water and 0.025M Sodium Chloride solution.

PROCEDURE

Experiment 1 :

1. Perform the general start-up as in section 4.1 2. Open valve V9 and switch on pump P1 3. Adjust P1 floe controller to give a constant flow rate f de-ionized water into the reactor R1 at approximately 700ml/min at F1-01

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4. Let the de-ionized water to continue flowing through the reactor until the inlet (Q1-01) and the outlet (Q1-02) conductivity values are stable a low levels. Record both conductivity values. 5. Close valve V9 and switch off pump P1 6. Open valve V11 and switch on pump P2. Start the timer simultaneously 7. Adjust pump P2 flow controller to give a constant flow rate of salt solution into the reactor R1 at 700ml/min at F1-02 8. Let the salt solution to flow for 1 minute, then reset and restart the timer. This will start the time at the average pulse input. 9. Close valve V9 and switch off pump P2. Then, quickly open valve V9 and switch on pump P1. 10. Make sure the de-ionized water flow rate is always maintained at 700ml/min by adjusting P1 flow controller 11. Start recording both the inlet (Q1-01) and outlet (Q1-02) conductivity values at regular intervals of 30 seconds. 12. Continue recording the conductivity values until all readings are almost constant and approach the stable low level values.

Experiment 2 :

1. Perform the general start-up as in section 4.1 2. Open valve V9 and switch on pump P1 3. Adjust P1 floe controller to give a constant flow rate f de-ionized water into the reactor R1 at approximately 700ml/min at F1-01 4. Let the de-ionized water to continue flowing through the reactor until the inlet (Q1-01) and the outlet (Q1-02) conductivity values are stable a low levels. Record both conductivity values. 5. Close valve V9 and switch off pump P1 6. Open valve V11 and switch on pump P2. Start the timer simultaneously 7. Record both the inlet (Q1-01) and outlet (Q1-02) conductivity values at regular intervals 30 seconds 6

8. Continue recording the conductivity values until all readings are almost constant.

RESULTS

Experiment 1: Pulse Input in a Turbular Flow Reactor Flow rate

= 700 mL/min

Input type

= Pulse input

Time

Conductivity

(min)

(mS/cm) Inlet

Outlet

C(t)

E(t)

tm

σ2

s3

Ci∆t

Ci(∆t)

t*E(t)/

(t - tm) 2 *

(t - tm) 3 *

E(t)/

E(t)/

∑Ci(∆t)

0.0

0.0

0.0

0.00

0.0000

0.0000

0.0000

0.0000

0.5

0.2

0.0

0.00

0.0000

0.0000

0.0000

0.0000

1.0

0.1

0.1

0.05

0.0217

0.0094

0.0093

0.0092

1.5

0.0

1.6

0.80

0.3478

0.2268

0.2451

0.3121

2.0

0.0

2.0

1.00

0.4348

0.3781

0.4973

0.8066

2.5

0.0

0.7

0.35

0.1522

0.1654

0.3607

0.8420

3.0

0.0

0.1

0.05

0.0217

0.0283

0.0833

0.2476

3.5

0.0

0.1

0.05

0.0217

0.0330

0.1134

0.3932

4.0

0.0

0.0

0.00

0.0000

0.0000

0.0000

0.0000

2.30

1.0035

0.8410

1.3091

2.6107

SUMMATION

Table 1

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C out (mS/cm) vs Time (min) 2.5

C out (mS/cm)

2 1.5 1

C out (mS/cm)

0.5 0 0

1

2

-0.5

3

4

5

Time (min)

Graph 1: outlet conductivity against time

E(t) 0.5 0.45 0.4 0.35

E(t)

0.3 0.25 E(t)

0.2 0.15 0.1 0.05 0 -0.05 0

0.5

1

1.5

2 2.5 Time (min)

3

3.5

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Graph 2: distribution of exit time, E(t) against time

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4.5

Experiment 2: Step Change Input in a Turbular Flow Reactor Flow rate

= 700 mL/min

Input type

= Step Change

Time

Conductivity

(min)

(mS/cm) Inlet

Outlet

C(t)

E(t)

tm

σ2

s3

Ci∆t

Ci(∆t)

t*E(t)/

(t - tm) 2 *

(t - tm) 3 *

E(t)/

E(t)/

∑Ci(∆t)

0.0

0.0

0.0

0.00

0.0000

0.0000

0.0000

0.0000

0.5

3.2

0.0

0.00

0.0000

0.0000

0.0000

0.0000

1.0

3.4

0.0

0.00

0.0000

0.0000

0.0000

0.0000

1.5

3.4

0.0

0.00

0.0000

0.0000

0.0000

0.0000

2.0

3.4

0.0

0.00

0.0000

0.0000

0.0000

0.0000

2.5

3.4

1.4

0.70

0.1818

0.1181

0.2679

0.6381

3.0

3.3

2.1

1.05

0.2727

0.2125

0.5504

1.5342

3.5

3.2

2.1

1.05

0.2727

0.2479

0.7491

2.4362

4.0

3.2

2.1

1.05

0.2727

0.2833

0.9785

3.6366

3.85

0.9999

0.8618

2.5459

8.2451

SUMMATION

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C out (mS/cm) vs Time (min) 2.5

C out (mS/cm)

2 1.5 1

C out (mS/cm)

0.5 0 0

1

2

-0.5

3

4

5

Time (min)

Graph 1: outlet conductivity against time

E (t) vs Time (min) 0.35 0.3 0.25

E (t)

0.2 0.15

E (t)

0.1 0.05 0 -0.05

0

0.5

1

1.5

2

2.5

3

3.5

4

Time (min)

Graph 2: distribution of exit time, E(t) against time

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4.5

SAMPLE OF CALCULATION ∫ Area = (0.0X0.0)+(0.0X0.5) +(0.1X0.5)+(1.6X0.5)+(2.0X0.5)+ (0.7X0.5)+(0.1X0.5) +(0.1X0.5)+(0.0X0.5) Area = 2.30

DISCUSSION In this experiment, the „Tubular Flow Reactor‟ was conducted to examine the effect of a pulse input and step change in a tubular flow reactor and also to construct a residence time distribution (RTD) function for the tubular flow reactor. De-ionized water and Salts solution was used as chemicals to determine the conductivity.

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To construct a residence time distribution (RTD) function for the tubular flow reactor with pulse input, first we find the area under the c curve by using a numerical evaluation of integrals. After that using formula

∫

, we find the

values of E(t). From that, we plot E(t) as a function of time. This is the residence time distribution (RTD) function for the plug flow reactor. On the other hand, we use graphical method to find E(t) for step input. The steps were more complex than the steps use in finding the area under curve. Besides that, there are high probabilities that some error may occur while trying to find the differentiation of conductivity when using this method. The step input is usually easier to carry out than the pulse test. However, step input involves differentiation of the data when to obtain the RTD. This may lead to large errors (Forgler, 2006). When comparing the RTD function plot between experiment 1 and 2, we can see the different between both graphs. The two most important parameters that characterize a curve in graph are the mean time which indicates when the wave of tracer passes the measuring point and the variances which indicates how much tracer has spread out during the measurement time (Coulson & Richardson, 1999). As we compare the pulse input and step change input, pulse input given higher mean time but lower variances. So, we can say that step change input help the tracer to spread out more amount that pulse input. As we see the time, pulse input process was faster than step change input. The advantages of using Tubular Flow Reactor are it has high volumetric unit conversion, it can run for a long period of time with maintenance and the heat transfer can be optimized by using tubes in parallel while the disadvantages of using this reactor are the temperatures are difficult to control, variability of products which mean the input product might be not the same output and high operating cost. The errors involved during this experiment were: 

We must open the valve and pump simultaneously for example valve V11 and pump P2. To open them simultaneously is a bit difficult for us.



During taken of time by using stopwatch, we cannot stop at accurate time. So, the data might be not consistent.

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CONCLUSION

From the results obtained and the sample calculation done, several graphs were plotted. 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 and de-ionized water was used. The summation of C(t) came to a result of 2.30, and the sum of E(t) came to a result of 1.0035. As seen from the graph, both outlet conductivity and E(t) remained constant until minute 1, then took a sharp increase until minute 2, and a big decline after. This means that a unit pulse response was recorded at the outlet stream, indicating a flow of conductivity. For experiment 2, the effect of a step change input was examined. The flow rate was also kept constant at 700 ml/min and de-ionized water was also used. The results and calculations show that the summation of the conductivity was 3.85 and the sum of E(t) was 0.9999. From the graphs plotted, it is seen that the data increases sharply after some time, and gradually increases after. The experiment was considered a success as all objectives were achieved.

RECOMMENDATIONS

There are a few recommendations for this experiment. Before starting the experiment, make sure that students read a lab manual and understand how to do. First, make sure all the apparatus is cleaned before use. Then, make sure the time taken to get the conductivity is 30 seconds and the flowrate is maintained a 700ml/min.

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REFERENCES

1. Smith, J.M., Chemical Engineering Kinetics, McGraw Hill Int., 1981. 2. Fogler, H.S., Elements of Chemical Reaction Engineering, Prentice-Hall Inc., USA, 2nd Ed, 1992. 3. Levenspiel, O., Chemical Reaction Engineering, Third ed.,John Wiley and Sons, New York, 1999. 4. Armfield CET-Tubular Flow Reactor, Instruction Manual, 1993.

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APPENDIX

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