Experiment 6 CSTR

CHE151-1L Chemical Engineering Laboratory 2 4th Quarter AY 2016-2017

Continuously-Stirred Tank Reactor (CSTR) Evora, Micaella Francesca 1, Gomez, Ricky Jay 1, Santiago, Camille1 1

Student, Mapúa University, School of Chemical Engineering and Chemistry

ABSTRACT Continuous-Stirred Tank Reactor consists of a well-stirred tank into which there is a continuous flow of reacting material. It is operated in steady-state a nd is assumed that it has a perfect mixing condition. A CSTR is different from a batch reactor in the fact that it is not closed when operating the mass flows in and out of t he system. In this experiment, the CSTR was used to calculate the reaction rate constant, the Arrhenius constant (A) and activation ener gy (E) of the reaction. From the results of t he experiment, it was observed that the specific reaction rate decreases as the temperature increases where in it should be increasing. From the graph, the activation energy calculated has a negative value and the value of the Arrhenius constant is too small. Based on the theoretical concepts, the activation energy should have a positive value and the value of Arrhenius constant should be logical enough. Possible sources of error are that the conditions of the equipment might not be well established that is why there are problems in data gathering. Keywords: CSTR, Arrhenius constant, activation energy

INTRODUCTION

Continuous-Stirred Tank Reactor (CSTR), shown in Figure 1, consists of a well-stirred tank into which there is a continuous flow of reacting material, and from which the  partially reacted material passes [1] continuously . This is because these kinds of vessels are squat in shape that are good stirring if their contents are essential. Otherwise there could be an occurrence of a  bulk streaming of the fluid between the inlet and outlet and much of the volume of the vessels would be essentially dead space. The CSTR configuration is widely used in industrial applications and in wastewater treatment units.

Figure 1. Line Diagram of a CSTR [1]

Stirring is an important characteristic of a CSTR. The most appropriate first approximation to an estimation of its  performance is based on the assumption that its contents are perfectly mixed. As a consequence the effluent stream has the same composition as the contents and it differentiates the CSTR and the tubular reactor. It is not difficult to attain the fair approximation to perfect mixing in a CSTR, given that the fluid phase is not too viscous[1]. A CSTR is operated at steady state where the accumulation is equal to zero and are assumed to be perfectly mixed [3]. The main assumption in this case is that the concentration of the incoming fluid will  become instantaneously equal to the outgoing upon entering the vessel. Figure 2 visually explains the concept. This makes the temperature, concentration, and reaction rate independent of position in the reactor.

rate of reaction, it will lead the following equation:

 = −− 

(2)

The CSTR design equation gives the reactor volume V necessary to reduce the entering flow rate of species j from F j0 to the exit flow rate F j, when species j is disappearing at a rate of – r j . Wherein, F j is equal to C j multiplied by v. And will obtain the ideal CSTR mole  balance equation that is algebraic and not a differential equation given in equation 3:

 = −− 

(3)

Furthermore, an advantage of a CSTR is its openness of the construction compared to the tubular reactor. It makes it very easy to clean the internal surfaces and this is important  because in the case of the reactions where there is a tendency for the solid matter to be deposited such as polymerization processes and reactions in which tarry material is a by product. The reaction:  NaOH + H3COOC2H5 +C2H5OH

Figure 2. Concept of a CSTR [2]

A CSTR differs from a batch only in the fact that it is not closed. Thus, the mass flows in and out of the reactor is given in equation 1:

 =   −  + ∫   =0

(1)

Since the CSTR is steady-state and has a  perfect mixing condition, accumulation is equal to zero and no spatial variations in the



CH3COONa

can be considered equimolar and first order with respect to both sodium hydroxide and ethyl acetate with a second order overall within the limits of concentration (0-0.1M) and temperature of 20oC to 40oC. The reaction carried out in a CSTR will eventually reach steady state when a certain amount of conversion amount of conversion of the starting reagents has taken place. Both sodium hydroxide and sodium acetate contribute conductance to the reaction solution whilst ethyl acetate and ethyl alcohol do not. The conductivity of a sodium

hydroxide solution at a given concentration and temperature, however, is not the same as that of a sodium acetate solution at the same molarity and temperature and a relationship has been established allowing conversion to  be inferred from conductivity. The purposes of the experiment are to use graphical analysis in calculating reaction rate constant at different temperatures and to calculate the Arrhenius constant (A) and activation energy (E) of the reaction. METHODOLOGY

Figure 4. Label of the Set-up of the CSTR [4]

Materials and Set-up

Experimentation

The equipment that was utilized for this experiment is a Continuous-Stirred Tank Reactor (CSTR), illustrated in Figure 3 and 4 which depicts its schematic diagram,  provided by Armfield. As for the chemicals involved 0.1 M of sodium hydroxide and 0.1M ethyl acetate with both contained of 2.5 litres each.

The reagents were filled in the apparatus by the laboratory assistant. The set point of the temperature controller was set to o  o approximately 30 C and then to 40 C. Every 5 seconds the conductivity data was recorded until it reached its steady-state condition in the reactor. The data were gathered at approximately 30 to 45 minutes. Both feed pumps and the agitator motor were switch on, and instigated the data logger  program. After a few minutes the temperature sensor tip is covered (about 25mm of liquid in reactor)  –   switch on the hot water circulator [4]. The Armfield data logger  provided the set of readings of conductivity with time and stored in an excel file. Figure 4 displays the Armfield Data Logger or software interface of the apparatus.

Figure 3. Set-up of the CSTR

dependence of the specific rate constant and the estimation of the value of specific rate constant at various temperature ranges.

According to the Arrhenius principle, as the temperature increases, the rate constant decreases. This can be seen when the plot of the natural logarithm of specific rate of Figure 5. Armfield Data Logger

reaction versus the inverse of temperature was done. The Arrhenius equation shown in

Treatment of Results

(4),

The degree of conversion of the constituents can be converted with the conductivity of the contents of the reactor that were recorded over a period of the reaction.

Arrhenius constant and the variables, k and T.

From the given data by the software, by graphical analysis, the rate constant can be solved and calculated.

correlates

relationship of the treated data from the experiment.

k = f(T) -1.13 0.0032 -1.14

0.00321 0.00322 0.00323 0.00324 0.00325

-1.15     k    n     l

-1.16 -1.17 -1.18 -1.19

RESULTS AND DISCUSSIONS

energy,

the variables mentioned. Figure 6 shows the

(4)

Where k is the rate constant, R is the gas constant, A is the Arrhenius constant, E is the activation energy and T is the temperature.

activation

The plot shows a linear relationship between

The calculation of the Arrhenius constant the following equation is to be utilized:

− +ln ln= 

the

-1.2

y = 1124.1x - 4.7883 R² = 0.6988 1/T

In this experiment, graphical analysis is

Figure 6. Estimation of k as a function of

incorporated in the estimation of the different

temperature (T = 40 oC).

 parameters such as the activation energy and Arrhenius constant (A). It is also used to

In this plot, as the reciprocal of temperature

generated a plot relating the temperature

increases, the value of the natural logarithm

of k also increases. Take into note that the

Table

1.  Calculated

Arrhenius equation gives the form where the

graphical method.

results

slope is the negative of E/R, the y-intercept

Ea

-10,177.2

 being the natural logarithm of A, natural

A

0.008327

from

the

J / mol

logarithm of k as y variable and reciprocal of temperature as x variable. From the curve

Another data interpretation done was the

fitting equation, the slope has a positive value,

determination of the temperature dependence

showing the direct relationship of the

of the specific reaction rate constant. Figure

variables. Theoretically, the value should be

2 shows the trend of the treated data from the

negative given the fact that rate constant

experiment.

decreases with the increase in temperature. Also, the trend of the curve is ambiguous and

Temperature Dependence of k

does not follow the theoretical assumptions and considerations.

0.325 0.320

Possible error for this part is that it might be

0.315     k

that the conditions of operation of the equipment is not established well, or it might also be a reason that there is a problem with

0.310 y = -0.0036x + 1.4423 0.305 R² = 0.6984 0.300 308.00000000 309.00000000 310.00000000 311.00000000 312.00000000 T,K

the

data

gathering

capability

of

the

equipment.

Figure 7. Plot of T versus k (T = 40 oC).

Ambiguous results also arise in this part,

Arrhenius principle shows that the specific

where the value of the activation energy has

reaction rate constant should be decreasing

a negative value and the value of the

with the increase in the temperature, that is

Arrhenius constant was too small, to the point

due to the form of the linear equation wherein

that having this certain value is not logical at

natural logarithm is incorporated. However,

all. Table 1 shows the calculated activation

in reality the reaction rate constant should

energy and the Arrhenius constant.

increase with the increase in the temperature. This is due to the improved activities of the molecules, due to the fact that increase in

temperature refers to the increase in energy

Temperature Dependence of k

contained by the molecules, and so the movements of the molecules should be rapid.

0.734 0.732 0.730

In Figure 7, opposite trend was observed from

the

theoretical

assumptions

0.728

and

0.726     k

considerations. Instead of specific reaction

0.724

rate increases, it actually decreases with the

0.720

increase in temperature. Again, possible source of error for this part are the conditions

0.722 0.718 0.716 299.00000000 299.10000000 299.20000000 299.30000000 299.40000000 T, K

or operation of the equipment, problems with the equipment itself or the calibration of the

Figure 9. Plot of T versus k (T = 30 oC).

equipment measurements. If given the opportunity of good data, the The previous figures were the results when

results should at least abide the theoretical

the set reactor temperature is 40 oC. For the

concepts that specific rate of reaction should

temperature of 30 oC, same trend of the data

increase with the increase in temperature,

was observed. These are shown in Figure 8

activation energy should have a positive

and Figure 9.

value and the value of Arrhenius constant should be logical enough.

k = f(T)

    k    n     l

-0.305 0.003340.003341 0.003342 0.003343 0.003344 0.003345 -0.31

CONCLUSION

-0.315

Continuous-Stirred Tank Reactor is widely

-0.32

used in industrial applications and in

-0.325

wastewater treatment units. Stirring is an

-0.33 -0.335

1/T

important characteristic of a CSTR. It is operated

Figure 8. Estimation of k as a function of

temperature (T = 30 oC).

at

steady

state

where

the

accumulation is equal to zero and is assumed to be perfectly mixed. The objectives of the experiment were obtained; the results of the

experiment were computed using graphical

 be positive and the value of the Arrhenius

representation. From the Arrhenius principle,

constant should be logical enough.

when the temperature increases then the rate constant decreases. It was observed that the reciprocal of temperature increases and the

REFERENCES

value of the natural logarithm of k also increase. The slope gives off a positive value

1] Denbigh, K. G., and Turner, J. C. R., Chemical Reactor Theory: An Introduction

wherein the value should be negative based on the Arrhenius principle. The calculated results using the graph were indefinite  because the computed activation energy is negative and the value of the Arrhenius constant was too small. In the graphical analysis of the temperature dependence, it follows the principle; however, in reality the reaction rate constant should increase with the increase in the temperature. This is due  because of the activities of the molecules, which the increase in temperature refers to the increase of the energy of the molecule. Possible errors in this experiment are that the conditions of the equipment might be not well established and it causes problem in terms of gathering data using the equipment. The

calibration

of

the

equipment

measurements also can be a source of error for this experiment. If the data obtained were good enough then the results can abide the theoretical concepts in which the specific rate of reaction should increase with an increase in temperature, the activation energy should

[2] Stenstrom, M.K. & Rosso, D., Fundamentals of Chemical Reactor Theory. 2003 [3] Levenspiel O., Chemical Reaction Engineering, 3 ed., 1999. [4] CEM MKII Manual Issue 14

APPENDIX Sample Computations:

1. Calculation of E.

ln = 1124.1 (1)−4.7883 -E/R = 1124.1 R = 8.314 J/ mol-K E = - 8.314 J/mol-K (1124.1) E = -10177.2 J/mol 2. Calculation of A ln A = -4.78883 A = exp (-4.78883) A = 0.008327 Treated Data:

0.003226

-1.164

0.003226

-1.15687

0.003227

-1.16283

0.003226

-1.15687

0.003226

-1.164

0.003224

-1.16634

0.003224

-1.16634

0.003224

-1.17341

0.003225

-1.16518

0.003223

-1.16751

0.003222

-1.16867

0.003223

-1.16751

0.003221

-1.16983

0.003222

-1.16159

0.003221

-1.13417

0.003221

-1.16983

0.00322

-1.17098

0.00322

-1.17098

0.00322

-1.16393

0.00322

-1.16393

0.00322

-1.17098

0.003221

-1.16277

0.003221

-1.16983

0.003218

-1.17328

0.003219

-1.17213

1/T

ln k

0.003218

-1.17328

0.003238

-1.14234

0.003218

-1.17328

0.003238

-1.14965

0.003218

-1.16626

0.003239

-1.14843

0.003218

-1.16626

0.003238

-1.14965

0.003216

-1.17556

0.003237

-1.15086

0.003217

-1.16741

0.003238

-1.14965

0.003217

-1.16741

0.003237

-1.15086

0.003217

-1.18139

0.003237

-1.14357

0.003217

-1.17442

0.003235

-1.14603

0.003216

-1.17556

0.003234

-1.14725

0.003215

-1.17669

0.003234

-1.14725

0.003217

-1.17442

0.003232

-1.15689

0.003216

-1.16857

0.003231

-1.16526

0.003216

-1.17556

0.00323

-1.16643

0.003215

-1.17669

0.003228

-1.16165

0.003215

-1.16972

0.003216

-1.18252

0.00321

-1.17541

0.003214

-1.18476

0.003211

-1.17428

0.003214

-1.17086

0.00321

-1.1892

0.003213

-1.17895

0.00321

-1.18232

0.003214

-1.17086

0.003214

-1.17783

0.003216

-1.16857

0.003214

-1.17783

0.003214

-1.17783

0.003214

-1.17783

0.003213

-1.17895

0.003213

-1.15095

0.003213

-1.172

0.003213

-1.17895

0.003214

-1.17086

0.003213

-1.172

0.003213

-1.172

0.003212

-1.16617

0.003212

-1.18008

0.003212

-1.17314

0.003212

-1.18698

0.003213

-1.17895

0.003212

-1.18008

0.00321

-1.18232

0.003212

-1.18698

0.003211

-1.1812

0.003213

-1.17895

0.003212

-1.18008

0.003212

-1.17314

0.003212

-1.17314

0.003211

-1.1812

0.003211

-1.1812

0.003212

-1.18008

0.003211

-1.17428

0.003211

-1.1812

0.003211

-1.18809

0.003212

-1.17314

0.003211

-1.1812

0.003209

-1.17654

0.00321

-1.17541

0.00321

-1.18232