tubular reactor bp101b (2).docx

Abstract For tubular reactor model BP 101-B, we carried out two experiments. In the first experiment, we have examined the effect of pulse input in a tubular flow reactor and we have constructed a residence time distribution (RTD) function for the tubular flow reactor. In this experiment the deionized water are let to continue flowing through the reactor until the inlet and outlet conductivity values are stable at low levels. We need to maintain flow rate of deionized water at approximately 700mL/min. Then, the salt solution is let to flow for 1 minute. Both the inlet and outlet conductivity values at the regular intervals of 30 seconds are recorded. In the second experiment, our aims are to examine the effect of step change in tubular flow reactor and we have constructed a residence time distribution (RTD) function for the tubular flow reactor. Only the deionized water is allowed to continue flowing until the inlet and outlet conductivity values are stable at low levels. We continued record the conductivity values until all readings are almost constant.

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Introduction In the majority of industrial chemical processes, the reactor is the key equipment in which raw materials undergo a chemical change to form desired products. The design and operation of chemical reactors is thus crucial to the whole success of an industrial process. Reactors can take a widely varying form, depending on the nature of the feed materials and the products. Understanding the behaviour of how reactors function is necessary for the proper control and handling of a reaction system. Basically, there are two main groups of reactors; batch reactors and continuous flow reactors. The tubular flow reactor is commonly used in industry in addition to the CSTR and batch reactor. It consists of cylindrical pipe and is normally operated at steady state. For analysis purposes, the flow in the system is considered to be highly turbulent and may be modelled by that of the plug flow. Thus, there is no radial variation in concentration along the pipe. The reactants are continuously consumed as they flow down the length of the reactor in the tubular reactor. Through the reactor, the concentration is assumed vary continuously in axial flow direction. Which then the reaction rate will also vary.

Residence time distribution factor The Residence time distribution factor (RTD) of a reactor is a characteristic of the mixing that occurs in the chemical reactor. In a plug flow reactor, there is no axial mixing. Thus the omission is reflected in the RTD exhibited by the reactors. 2

Aims To examine the effects of a pulse input in a tubular flow reactor To construct a residence time distribution (RTD) function for the tubular flow reactor

Theory

From figure above let say, v0= FAo To develop the tubular flow reactor (TFR) design equation, then reactor volume shall be divided into a number of subvolumes so that within each subvolume

, the reaction may be

considered spatially uniform. Assuming that subvolume is located a distance y from the entrance of the reactor, then FA(y) is the molar flow rate of A into volume is the molar flow rate of A out of the volume. In spatially uniform subvolume ∫

3

and FA(y +

)

For tubular reactor at steady state, the general mole balance is reduced to,

FA(y) - FA(y +

)+r

=0

eq(1)

In the above expression, rA is an indirect function of y.That is, rA is a function of reactant concentration, which is a function of the position, y down the reactor. The volume,

is the

product of the cross- sectional area, A of the reactor and the reactor length , ∆y. eq(2) Substituting equation (2) into equation (1) yields,

-

(

)

( )

] = -ArA

Taking the limit as ∆y approaches zero, (

(

)

( )

)

It is usually most convenient to have the reactor volume, V rather than the reactor length, y as the independent variable. Accordingly, the variables ‘Ady’ can be changed to dV to obtain this form of the design equation for a TFR. Note that for reactor a reactor in which the cross- sectional in which the crosssectional area, A varies along the length of the reactor, the design equation remains unchanged. This means that extent of reaction in a plug flow reactor does not depend on its shape, but only on its total volume. If FAO is the molar flow rate of species A fed to a system operated at steady state, the molar flow rate at which species A is reacting within the entire system will be [FAOX].The molar feed rate of A to the system minus the rate of reaction of within the system equals the molar flow rate of A leaving the system, FA. This shown in mathematical form to be, FA.= FAO - FAOX = FAO (1-X) The entering molar flow rate FAO is just the product of the entering concentration C AO and the entering volumetric flow rate Vo

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FAO = CAOVo Combining above equations yields the design equation with conversion term for the TFR, FAO Rearranging and integrating above equation with the limit V=0 and X=0, we obtain the plug flow reactor volume necessary to achieve a specified conversion X, V=FAO∫ The residence time distribution (RTD) of reactor is a characteristic of the mixing that occurs in the chemical reactor. There is no axial mixing in a plug flow reactor (PFR), and this omission is reflected in the RTD which is exhibited by this class of reactors. The continuous stirred tank reactor (CSTR) is thoroughly mixed and possesses a far different kind of RTD than the PFR. Not all RTDs are unique to a particular reactor type; markedly different reactors can display identical RTDs. Nevertheless, the RTD exhibited by a given reactor yields distinctive clues to the type of mixing occurring within it and is one of the most informative characterizations of the reactor. The RTD is determined experimentally by injecting an inert chemical called a tracer into the reactor at some time t=0 and then measuring the tracer concentration, C in the effluent stream as function of time. The two most methods of injection are pulse input and step input.

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Pulse input In a pulse input, an amount of tracer, No is suddenly injected in one shot into the feed stream entering the reactor in as short time as possible. The outlet concentration is then measured as function of time. The effluent concentration vs. time is referred to as C(t) curve in RTD analysis. Consider the injection of a tracer pulse for single input and single output system in which only flow (no dispersion) carries the tracer material across system boundaries. For a small time increment t (sufficiently small so that the concentration of tracer is essentially constant during that time period), the amount of tracer C(t) exiting between time t and (t + ∆t) is, ∆N= C(t)v∆teq(3) where v is the effluent volumetric flow rate. In other words, ∆N is the amount of material that has spent an amount of time between t and (t + ∆t) in the reactor. If it is divided by total amount of material that was injected into the reactor, No ( )

which represent the fraction of the material that has residence time in the reactor between time t and (t + ∆t). For pulse injection, E(t) =

( )

eq(4) ( )

The quantity E(t) is called the residence time distribution function. It is the function that describes in a quantitative manner how much time different fluids elements have spent in the reactor.

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If No is not known directly, it can be obtained from the outlet concentration measurement by summing up all the amounts of materials,

between time t = 0 and infinity. Equation (3)

can be written in differential form, dN= C(t)vdteq (5) Integrating equation (5) gives ( )

No = ∫

The volumetric flow rate, v is usually constant, so that by substituting eq(5) into eq(4) we can define E(t) as, E(t) =

( ) ∫

( )

The integral in the denominator is the area under the C(t) curve.

Apparatus SOLTEQ Tubular Reactor (Model: BP 101-B), Conical flask, Calibration meter, Sodium hydroxide, NaOH (0.1M) Sodium Acetate, Na(Ac) (0.1M) Deionised water, H2O

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Procedure Pulse input in a tubular flow reactor 1. The general start up procedure is set up. 2. V9 is opened and pump P1 is switched on. 3. The flow controller pump P1 is adjusted to give a constant flow rate of deionised water into the reactor R1 at approximately 700 ml/min at F1-01. 4. The deionized water is let to flow continuously through the reactor until the inlet (Q101) and outlet (Q1-02) conductivity values are stable at low levels. Both conductivity value are recorded. 5. Valve V9 is closed and the pump P1 is switched off. 6. Valve V11 is opened and the pump P2 is switched on. The timer start simultaneously. 7. The flow controller pump P2 is adjusted to give a constant flow rate of salt solution into the reactor R1 at 700 ml/min at F1-02. 8. The salt solution is let to flow for 1 minute, then the timer is reset and restarted. 9. Valve V11 is closed and pump P2 is switched off. Valve V9 is quickly opened and the pump P1 is switched on. 10. The deionised water flow rate is always maintained at 700 ml/min by adjusting the P1 flow controller. 11. Both the inlet (Q1-01) and outlet (Q1-02) conductivity values is recorded at regular intervals of 30 seconds. 12. The conductivity value is recorded continuously until all readings are almost constant and approached stable low level values.

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Results Flow rate

:

Input type

:

ml/min

Time (min)

Conductivity (mS/cm) inlet

Outlet

0.0

0.0

0.0

0.5

0.0

0.2

1.0

0.0

2.7

1.5

0.0

4.7

2.0

0.0

4.8

2.5

0.0

2.5

3.0

0.0

0.6

3.5

0.0

0.2

4.0

0.0

0.1

4.5

0.0

0.0

5.0

0.0

0.0

5.5

0.0

0.0

Outlet conductivity (mS/cm) vs. Time (min) 6

Outlet conductivity (mS/cm)

5 4 3 2 1 0 0 -1

1

2

3 Time (min)

9

4

5

6

Time (min)

Outlet conductivity E(t) (mS/cm)

0

0

0

0.5

0.2

0.01766

1

2.7

0.238411

1.5

4.7

0.415011

2

4.8

0.423841

2.5

2.5

0.220751

3

0.6

0.05298

3.5

0.2

0.01766

4

0.1

0.00883

4.5

0

0

5

0

0

5.5

0

0

Residence time distribution (RTD) function for plug flow reactor

E(t) vs. time (min) 0.5 0.45 0.4 0.35

E(t)

0.3 0.25 0.2 0.15 0.1 0.05 0 -0.05 0

1

2

3 Time (min)

10

4

5

6

Sample calculations ∫

( )

(

∫

∫

( )

∫

( )

∫

( )

(

)

( )

(

(

( )

∫

)

(

)

( )

∫

(

)

(

)

(

)

t=2.0, C(t)=4.8 ( )

( ) ∫

( )

( )

t=2.5, C(t)=2.5

t=0, C(t)=0

( )

( )

t=3.0, C(t)=0.6

t=0.5, C(t)=0.2

( )

( )

t=3.5, C(t)=0.2

t=1.0, C(t)=2.7

( )

( )

t=4.0, C(t)=0.1

t=1.5, C(t)=4.7

( )

( )

11

)

(

))

Outlet

time

conductivity E(t)

(min)

( )

0

0

0

0.5

0.2

0.01766

0.00883

0.004415 0.002208

1

2.7

0.238411 0.238411 0.238411 0.238411

1.5

4.7

0.415011 0.622517 0.933775 1.400662

2

4.8

0.423841 0.847682 1.695364 3.390728

2.5

2.5

0.220751 0.551876 1.379691 3.449227

3

0.6

0.05298

0.15894

0.476821 1.430464

3.5

0.2

0.01766

0.06181

0.216336 0.757174

4

0.1

0.00883

0.03532

0.14128

(

( )

( )

)

(

)

( )

σ2= (3.404+0.1236)-

)

(

)

(

)

(

)

(

)

(

)

)

(

(

(

(

))

(

(

0.565121

( )

∫ (

( ∫

0

0

Second moment, Variance, σ2=∫ ( ∫

t^3 E(t)

0

( ∫

t^2 E(t)

(mS/cm)

Mean residence time, ∫

tE(t)

)

) (

( ) )

)) (

)

=0.2768min2 12

∫ (

Third moment, Skewness, s3=

∫

( )

(

(

S3=(

( )

(

( )

)

))

( ∫

)

)

(

(

)

)

13

(

)

(

)

(

)

Discussion The objectives in this tubular reactor experiment is to examine the effects of a pulse input in a tubular flow reactor and also to construct a residence time distribution (RTD) function for the tubular flow reactor. 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. 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 in the ideal tubular reactor. Tubular reactors resemble batch reactors in providing initially high driving forces, which diminish as the reactions progress down the tubes. In the experiment we are examining the effects of pulse input, in which the flow rate of the deionized water is kept constant at 700 ml/min. The graph of the outlet conductivity (mS/cm) vs time (min) has been plotted. At the time 2 minutes the outlet conductivity that we get is 4.8 mS/cm which is the highest value. After that the conductivity decreases within the time. From the graph we can see that the outlet conductivity reaching zero value at 4.5 minutes which are not much differ from the theory where it should be 4 minutes .So we conclude for the first part of our experiment is succeed. Next, the residence time distribution can be determined by constructing a residence time distribution (RTD) function for this experiment. In order to get the residence time, the C(t) curve is very useful to construct the RTD . The graph of E(t) vs time (min) has been plotted. As the curve of this RTD is affect by the conductivity value, thus the value of the residence time is highest when at time 2 minute and also this curve reaching zero value at time 4.5 minute. The mean residence time is calculate from the graph, and then the variance (second moment) and skewness (third moment) also been calculate. The values are 1.802 minutes, 0.2768min2 and 1.5805 respectively.

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The tubular flow reactor have both advantages and disadvantages. The advantages of this flow reactors are its have a high volumetric unit conversion, can be run for long periods of time without maintenance, and the heat transfer rate can be optimized by using more, thinner tubes or fewer, thicker tubes in parallel. While the disadvantages of this flow reactors are that temperatures are hard to control and can result in undesirable temperature gradients. PFR maintenance is also more expensive than continuous stirred tank reactor (CSTR) maintenance. Tubular flow reactor is mainly used for large scale reactions, fast reactions, homogeneous or heterogeneous reaction, and continuous production and also for the high temperature reactions.

Conclusion As the conclusion, the objective to examine the effect of pulse input in the tubular reactor is achieved. The time that we get when it’s reaching the zero value which is at 4 minutes show that our experiment is succeed as it’s seem similar to the theoretical and lastly we also managed to construct a residence time distribution (RTD).Where the mean residence time, tm value is 1.802 minutes. The second moment, variance σ2 value is 0.2768min2 and the third moment, skewness, s3 value is 1.5805.

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Recommendations After we have finished this experiment, we find that there are several factors in this experiment that can be fixed to make sure that the experiments runs better. Below is some of the recommendation for this experiment runs better. 1. Always check and rectify any leak at the reactor. 2. Make sure the conductivity of the inlet and outlet stable before start the experiment. 3. The flow rate must be kept constant. 4. All of the valve must be open fully to ensure constant flow. 5. The conductivity value must be taken until three constant value.

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Reference 1. Webs : 

Retrieved on 4th April 2012 at 0850, http://www.metal.ntua.gr/~pkousi/elearning/bioreactors/page_07.html



Retrieved

on

4th

April

2012

at

0850,

http://www.scribd.com/alipjack/d/52314559/3-EXPERIMENT-3-PLUGFLOW-REACTOR-PFR 

Retrieved

on

7th

April

2012

at

2300

http://www.parrinst.com/products/specialty-custom-systems/5400continuous-flow-tubular-reactors/ 2. Books : 

Robert H.Perry, Don W.Green, Perry’s Chemical Engineers’ Handbook, McGraw Hill, 1998.



H. Scott Fogler, Elements of Chemical Reaction Engineering, 4th Edition, Pearson Education International, 2006.

Appendix Next page

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