UNIVERSITI TEKNOLOGI MARA MALAYSIA

FACULTY OF CHEMICAL ENGINEERING

PROCESS ENGINEERING LABORATORY II

NAME : AIMULLAH BIN RAZAK

STUDENT ID : 2007269622

EXPERIMENT : CONTINUOUS STIRRED TANK REACTOR (CSTR)

DATE PERFORMED : 18TH JANUARY 2008

PROG/CODE : Bachelor of Engineering (Hons.) in Process Engineering/EH221

NO Title Allocated Marks % Marks %1 Summary 52 Introduction 53 Objectives 54 Theory 55 Procedures 36 Apparatus 57 Results 208 Calculation 109 Discussion 2010 Conclusion 1011 Recommendation 512 References 513 Appendices 2

TOTAL 100

Remarks:

Checked by: Rechecked by:

Cik Radziah

TABLE OF CONTENT

CONTENTS PAGE

SUMMARY 3

INTRODUCTION 4

OBJECTIVES 4

THEORY 5

PROCEDURES 7

APPARATUS 8

RESULTS 10

CALCULATIONS 11

DISCUSSIONS 13

CONCLUSIONS 15

RECOMMENDATIONS 16

REFERENCES 16

APPENDICES 17

2

SUMMARY

In chemical engineering, chemical reactors are vessels designed to contain

chemical reactions. The reactor is the equipment in which empirical information is

obtained can be divided into two types, the batch and flow reactors. The batch reactor is

simply a container to hold the contents while they react. The flow reactor is used

primarily in the study of the kinetics of heterogeneous reactions. In this case of study, the

continuous stirred tank reactor is carried out in order to determine the order of the

saponification reaction and also to determine the reaction rate constant, k. According to

the theory, the saponification process can be categorized as the second order reaction. A

plot of the experimental data should be a straight line in which its slope is the rate

constant of the reaction. The deviation of graph is due to the lateness addition of sample

into HCl that will quench the reaction between sodium hydroxide and ethyl acetate. The

deviation of graph between minute 0 to minute 1 is also due to the titration process in

which the pink color that needs to obtain is not as light as needed. Also, the error when

taking the buret readings can be lead to the deviation when plotting both the graph of ln

CA versus time and ln CB/CA versus time.. The objectives of study are not successfully

achieved.

3

INTRODUCTION

Every industrial chemical process is designed to produce economically a desired

product from a variety of starting materials through a succession of treatment steps. The

raw materials undergo a number of physical treatment steps to put them in the form in

which they can be reacted chemically. They then pass through the reactor. The products

of the reaction must then undergo further physical treatment like separations, purification,

etc for the final desired product to be obtained.

In chemical engineering, chemical reactors are vessels designed to contain

chemical reactions. The design of a chemical reactor deals with multiple aspects of

chemical engineering. Chemical engineers design reactors to maximize the net present

value for the given reaction. Designers ensure that the reaction proceeds with the highest

efficiency towards the desired output product, producing the highest yield of product

while requiring the least amount of money to purchase and operate.

Reactor design uses information, knowledge and experience from a variety of

areas such as thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass

transfer and economics. Chemical reaction engineering is the synthesis of all these factors

with the aim of properly designing a chemical reactor.

The reactor is the equipment in which empirical information is obtained can be

divided into two types, the batch and flow reactors. The batch reactor is simply a

container to hold the contents while they react. The flow reactor is used primarily in the

study of the kinetics of heterogeneous reactions.

OBJECTIVES

The purposes of this Continuous Stirred Tank Reactor experiment are:

To determine the order of saponification reaction

To determine the reaction rate constant, k by plotting the graph

4

THEORY

The continuous stirred-tank reactor (CSTR), also known as vat- or backmix reactor is

a common ideal reactor type in chemical engineering. A CSTR often refers to a model is

used to estimate the key unit operation variables when using a continuous agitated-tank

reactor to reach a specified output. A stirred tank reactor (STR) may be operated either as

a batch reactor or as a steady-state flow reactor (better known as Continuous Stirred tank

Reactor {CSTR}). The key or main feature of this reactor is that mixing is complete so

that properties such as temperature and concentration of the reaction mixture are uniform

in all parts of the vessels. Material balance of a general chemical reaction is described

below:

The conservation principle requires that the mass of species A in an element of reactor

volume ΔV obeys the following statement:

[ Rate of A into ] – [ Rate of A out of ] + [ Rate of A produced ] = [ Rate of A ] volume element volume element within volume element accumulated within volume element

Batch Stirred Tank Reactor (BSTR)

In batch reactions, there are no feed of exits streams and therefore the general equation

can be simplified into:

[Rate of A produced in volume element] – [Rate of A accumulated in volume element]

The rate of reaction of component A is defined as:

-rA = 1 dNA = [moles of A appear by reaction] / [unit volume] [unit time] V dt by reaction

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By this definition, if A is a reaction product, the rate is positive; whereas if it is a reactant

which is consumed, the rate is negative.

(- rA) V = NAO dXA

dt

t = NAO ∫ dXA

(-rA)V

Steady State Mixed Flow Reactor

The general material balance for this reactor is:

Input of A (moles/time) = FAO (1-XAO) = FAO

Output of A (moles/time) = FA = FAO ((1-XA)

Dissapearance of A by reaction (moles/time) = (-rA) V

FAO XA = (-rA ) V

V = τ = ΔXA = XA

FAO CAO -rA -rA

τ = 1 = V = VCAO = CAO XA

S Vo FAO -rA

6

PROCEDURE

Experiment: Batch Reactor Experiment

Reactant Preparation Procedure

1. 0.05 M NaOH and 0.05 M Ethyl Acetate solutions is prepared in two separate 20

liter feed tanks.

2. The concentration of 0.1 M NaOH solution is to be confirmed by titrating a small

amount of it with 0.1 M HCl using phenolphthalein as indicator. The concentration of

ethyl acetate solution is evaluated by the following manner. First, 0.1 M NaOH solution

is added to a sample of the feed solution such that the 0.1 M NaOH solution is in excess

to ensure all of the ethyl acetate has reacted. This mixture is let to be reacting overnight.

On the following day, the amount of unreacted NaOH is determined by direct titration

with standard 0.1 M HCl. The ethyl acetate real concentration is then recorded.

3. 1 liter of quenching solution of 0.25 M HCl and 1 liter of 0.1 M NaOH is prepared for

back titration.

Batch Reaction Procedure

1. The overflow tube in the reactor is adjusted to give a desired working volume (2.5L).

The pump P1 is switched on and 1.25L of the 0.1M ethyl acetate is pumped from the feed

tank into the reactor. The pump P1 is stopped.

2. The pump P2 is switched on and 1.25L of the 0.05 M NaOH is pumped into the

reactor. The pump P2 is stopped when 2.5L volume is reached. The stirrer is switched on

and the speed is set to be 180 rpm. Immediately, the timing of the reaction is recorded.

3. 10mL of 0.25 M HCl is quickly measured in a flask

4. After 1 minute of reaction, the valve V7 is opened. 50mL sample is then collected and

added immediately to the 10mL of 0.25 M HCl prepared in step 3 and let to be mixed.

The HCl will quench the reaction between ethyl acetate and sodium hydroxide.

5. The mixture is then titrated with 0.1 M NaOH to evaluate the amount of unreacted

HCl.

6. Steps 3 to 5 is repeated for reaction times 5,10,15,20 and 25 minutes

7

APPARATUS

Chemicals

HCL, NaOH, Ethyl Acetate and phenolphthalein.

Equipment Description:

(a) Reactor

The reactor consists of a glass vessel with top and bottom plate made of stainless steel.

The reactor comes with a coiling coil, a 1.0kW heater, a temperature sensor, stirrer

system, an overflow tube and a gas sparging unit.

(b) Feed Inlet System (Liquid Reactions)

For each liquid reactant, a 20 L feed tank, a pump, a needle valve and a flow meter

are provided. Each reactant is pumped from the feed tank to the appropriate inlet port

at the top plate

(c) Gas Inlet System (Gas-Liquid Reactions:Optional)

Gas from gas cylinder flows through a needle valve, a flow meter and a gas sparger

before reacting with the liquid inside the reactor.

(d) Product/Waste Tank

A 50L rectangular tank made of stainless steel is provided for collecting the product or

waste before being discharged.

(e) Control Panel

The control panel consists of all the necessary electrical components for controlling

the operations of the unit. Components mounted on the panel door are all labeled for

convenience. The control panel also houses the necessary modules for data acquisition

system

(f) Data Acquisition System (Optional)

The Data Acquisition System consists of a personal computer, ADC modules and

instrumentations for measuring the process parameters. A flow meter with 0 to 5 VDC

output signal is supplied for each feed stream. A temperature sensor and temperature

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transmitter with 4 to 20 mA output signal is provided to measure the reaction

temperature. A pH sensor with controller are provided for monitoring the changes in

pH while the reaction is taking place. All analog signals from the sensors are converted

by the ADC modules into digital signal before being sent to the personal computer for

display and manipulation.

Figure 1: Continuous Stirred Tank Reactor Equipment

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RESULTS

Feed Concentration Concentration of NaOH (CNaOH) = 0.05 mol/L

Concentration of ethyl acetate (CEA) = 0.1 mol/L

Standard Solution Concentration of HCl (CHCl,std) = 0.25 mol/L

Volume of HCl (VHCl) = 10 mL = 0.01 L

Concentration of NaOH (CNaOH) = 0.1 mol/L

Sample Volume of Sample (VS) = 50 mL = 0.05 L

Time (min) 1 5 10 15 20 25

Volume of titrating NaOH (mL) 13.4 9.4 7.3 6.7 3.7 2.5

Volume of quenching HCl unreacter with NaOH in

Sample (mL)

5.36 3.76 2.92 2.68 1.48 1.00

Volume of HCl Reacted with NaOH in Sample (mL) 4.64 6.24 7.08 7.32 8.52 9.00

Mole of HCl Reacted with NaOH in Sample (mol) (x10-4) 11.6 15.6 17.7 18.3 21.3 22.5

Mole of NaOH unreacted in Sample (mol) (x10-4) 11.6 15.6 17.7 18.3 21.3 22.5

Concentration of NaOH unreacted with Ethyl Acetate

(mol/L) (X10-2)

2.32 3.12 3.54 3.66 4.26 4.50

Steady State Fraction Conversion of NaOH 0.536 0.376 0.292 0.268 0.148 0.10

Concentration of NaOH reacted with Ethyl Acetate

(mol/L) (X10-2)

2.68 1.88 1.46 1.34 0.74 0.50

Mole of NaOH reacted with Ethyl Acetate in Sample(mol)

(X10-3)

1.34 0.94 0.73 0.67 0.37 0.25

Concentration of Ethyl Acetate reacted with NaOH

(mol/L) (X10-2)

2.68 1.88 1.46 1.34 0.74 0.50

Concentration of Ethyl Acetate unreacted (mol/L) (X10-2) 7.32 8.12 8.54 8.66 9.26 9.50

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SAMPLE OF CALCULATIONS

(A) Sample: t = 1 min

(B) Volume of titrating NaOH (mL) = 13.4 mL = 0.0134L

(C) Volume of quenching HCl Unreacted = C NaOH,std x (B) with NaOH in Sample (mL) C HCl, std

= 0.1 mol / L x 13.4 mL 0.25 mol/L = 5.36 mL

(D) Volume of HCl Reacted with NaOH = VHCl – (C) in sample (mL) = 10 mL – 5.36 mL

= 4.64 mL = 0.00464L

(E) Mole of HCl Reacted with NaOH in = C HCl, std x (D) sample (mol) = 0.25 mol/L x 0.00464 L

= 11.6 x 10 -4 mol

(F) Mole of NaOH unreacted in Sample = (E) (mol)

= 11.6 x 10 -4 mol

(G) Concentration of NaOH unreacted = (E) with Ethyl Acetate (mol/L) VS

= 11.6 x 10 -4 mol 0.05 L

= 0.0232 mol/L

(H) Steady State Fraction Conversion = 1 – CA

of NaOH (XA) CAO

= 1 - 0.0232 mol/L 0.05 mol/L

= 0.536

(I) Concentration of NaOH reacted = CNaOH,O – (G) with Ethyl Acetate (mol/L)

= 0.05 mol/L - 0.0232 mol/L = 0.0268 mol/L

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(J) Mole of NaOH reacted with = (I) x VS

Ethyl Acetate in Sample(mol) = 0.0268 mol/L x 0.05 L = 1.34 x 10 -3 mol

(K) Concentration of Ethyl Acetate = (J) reacted with NaOH (mol/L) Vs

= 1.34 x 10 -3 mol 0.05 L

= 0.0268 mol/L

(L) Concentration of Ethyl Acetate = CEA,O – (K) unreacted (mol/L)

= 0.1 mol/L – 0.0268 mol/L = 0.0732 mol/L

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DISCUSSION

The continuous stirred tank reactor study is carried out experimentally in order to

determine the order of saponification reaction and also to determine the reaction rate

constant, k. The saponification process is the hydrolysis of an ester under basic

conditions to form an alcohol and the salt of a carboxylic acid .For example in this case

of study,

CH3COOC2H5 + NaOH → CH3COONa + C2H5OH

From the theory, the saponification process is known to be the first order reaction

in which, when the graph of ln CA versus time (min) is plotted, it will shows the straight

line which the slope is in negative value and is equal to the rate constant of the process.

The experimental data is summarized as below:

Time (min) 1 5 10 15 20 25CA 0.0232 0.0312 0.0354 0.0366 0.0426 0.0450

ln CA -3.764 -3.467 -3.341 -3.308 -3.156 -3.101

Figure 2: Graph ln CA versus time (min) (based on experimental data)

From Figure 2 above, the slope of the graph in which is known as the rate constant

of the process can be calculated as below:

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k = -3.308 – (-3.764) 15-1

k = 0.0326 min -1

The graph above shows the straight line but the rate constant was in a positive value.

According to the theory, the graph for the first order of saponification process needs to be

a straight line and positive value of slope which is the rate constant of the reaction.

Therefore, the graph above is altered so that the reaction between minute 0 to minute 1 is

not taken as consideration because of the minor error.

The theory also state that the saponification process is known to be the second order

reaction in which, when the graph of ln CB/CA versus time (min) is plotted, it will shows

the straight line which the slope is in positive value and is equal to the rate constant of the

process. The experimental data is summarized as below:

Time (min) 1 5 10 15 20 25CA 0.0232 0.0312 0.0354 0.0366 0.0426 0.0450CB 0.0732 0.0812 0.0854 0.0866 0.0926 0.0950

ln CB/CA 1.149 0.956 0.881 0.861 0.776 0.747

Figure 2: Graph ln CB/CA versus time (min) (based on experimental data)

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From Figure 3 above, the slope of the graph in which is known as the rate constant of the

process can be calculated as below:

(0.1 - 0.05)k = 0.776 – 0.956 20-5

k = -0.24 min -1

The graph above shows the straight line but the rate constant was in a negative value.

According to the theory, the graph for the second order of saponification process needs to

be a straight line and negative value of slope which is the rate constant of the reaction.

Therefore, the graph above is altered so that the reaction between minute 0 to minute 1 is

not taken as consideration because of the minor error. Also the result that we get does not

satisfy the theory due to human error during the experiment was conducted.

CONCLUSION

The continuous stirred tank reactor (CSTR) study is carried out experimentally

using the continuous stirred tank reactor equipment. The first objective of this study is to

determine the order of saponification reaction. The second objective is to determine the

rate constant of the reaction which can be calculated from both the slope of the graph of

ln CA versus time (min) and ln CB/CA versus time (min). Based on the result that we

obtain, the order of saponification reaction cannot be determined. Further more; the rate

of reaction that we calculate in the discussion part cannot be used due to the error in

collecting data from the experiment. The deviation of graph in Figure 2 and Figure 3 is

due to the lateness addition of sample into HCl that will quench the reaction between

sodium hydroxide and ethyl acetate. The deviation of graph between minute 0 to minute

1 is due to the titration process in which the pink color that needs to obtain is not as light

as needed.

15

RECOMMENDATION

Here are some recommendations regarding to this study:

The eyes should be meniscus to get the accurate readings in order to avoid

parallax error.

The titration process should be done slowly in order to get the first point in which

the light pink of sample is obtained.

The sample must be quickly added into HCl to avoid any experimental error due

to the lateness addition of HCl.

Glove and goggle should be wearing when dealing with the chemicals such as

hydrochloric acid, sodium hydroxide etc.

When taking the buret readings, white paper should be used in order to reduce

parallax error.

REFERENCES

1. Schmidt, Lanny D. (1998). The Engineering of Chemical Reactions. New York:

Oxford University Press.

2. Kenneth A. Connors Chemical Kinetics, the study of reaction rates in solution,

1991, VCH Publishers.

3. http://www.chem.arizona.edu/~salzmanr/480a/480ants/chemkine.html.

4. Isaacs, N.S., "Physical Organic Chemistry, 2nd edition, Section 2.8.3, Adison

Wesley Longman, Harlow UK, 1995.

16

APPENDICES

A batch stirred tank reactor is the simplest type of reactor.  It is composed of a reactor and a mixer such as a stirrer, a turbine wing or a propeller. The batch stirred tank reactor is illustrated below:

        This reactor is useful for substrate solutions of high viscosity and for immobilized enzymes with relatively low activity. However, a problem that arises is that an immobilized enzyme tends to decompose upon physical stirring. The batch system is generally suitable for the production of rather small amounts of chemicals.

        A continuous stirred tank reactor is shown below:

        The continuous stirred tank reactor is more efficient than a batch stirred tank reactor but the  equipment is slightly more complicated

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Continuous Stirred Tank Reactors (CSTRs)

Type of Reactor Characteristics

Continuously Stirred Tank Reactor (CSTR)

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

Kinds of Phases Present

Usage Advantages Disadvantages

1. Liquid phase

2. Gas-liquid rxns

3. Solid-liquid rxns

1. When agitation is required

2. Series configurations for different concentration streams

1. Continuous operation

2. Good temperature control

3. Easily adapts to two phase runs

4. Good control

5. Simplicity of

1. Lowest conversion per unit volume

2. By-passing and channeling possible with poor agitation

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construction

6 Low operating (labor) cost

7. Easy to clean

General Mole Balance Equation

Assumptions

1) Steady state therefore

2) Well mixed therefore rA is the same throughout the reactor

Rearranging the generation

In terms of conversion

19

Reactor Sizing

Given –rA as a function of conversion, –rA = f(X), one can size any type of reactor. The volume of a CSTR can be represented as the shaded areas in the Levenspiel Plot shown below:

Reactors in Series

Given –rA as a function of conversion, , –rA = f(X), one can also design any sequence of reactors in series provided there are no side streams by defining the overall conversion at any point.

Mole Balance on Reactor 1

20

Mole Balance on Reactor 2

Given –rA = f(X) the Levenspiel Plot can be used to find the reactor volume

For a PFR between two CSTRs

21

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