Cstr 40L

ABSTRACT

The experiment involves a continuous stirred tank reactor (cstr). This experiment is to

determine the effect of temperature onto the reaction extent of conversion and to determine

the reaction’s activation energy. Firstly the reactor is filled with sodium hydroxide, NaOH

and Ethyl acetate, Et(Ac) until achieved 10 L. The flow rate of both solutions must same at

0.20 L/min. The solution is stirred and speed of 200 rpm. The water temperature is set at 50

ºC. The steady state conductivity and temperature values is recorded and then find the

concentration of NaOH in reactor and the extent of conversion from calibration curve. The

50ml sample is collected then carried out the back titration procedure immediately. The

experiment is repeated with different temperature; 55 ºC, 60 ºC and 75 ºC.

OBJECTIVES

-To carry out a saponification reaction between NaOH and Et(Ac) in CSTR 40 litre.

-To determine the effect of residence time onto the reaction extent of conversion.

-To determine the rate constant.

-To determine the reaction rate.

INTRODUCTION

Reactor is one of equipment used mostly in the industrials sector. It changes the raw material

into the desired product. A good reactor will give a high production and economical. One of

criteria to choose or to design a good reactor is to know the effectiveness of the reactor itself.

There a many types of reactor depending on the nature of the feed materials and products.

One of the most important we need to know in the various chemical reaction was the rate of

the reaction. The continuous stirred-tank reactor (CSTR) which is also known as vat- or back-

mix reactor usually is a common ideal reactor type in chemical engineering. A CSTR often

refers to a model used to estimate the key unit operation variables when using a continuous

agitated-tank reactor to reach a specified output. This reactor works for all fluids, liquids,

gases, and slurries. The behaviour of a CSTR is always modelled by that of a Continuous

Ideally Stirred-Tank Reactor (CISTR). All calculations performed with CISTRs assume

perfect mixing. In a perfectly mixed reactor, the output composition is identical to

composition of the material inside the reactor, which is a function of residence time and rate

of reaction. CSTR used in this experiment, (model: BP 143) is designed for student’s

experiments on chemical reaction in liquid phase under adiabatic and isothermal conditions.

CSTR consists of two tanks of solutions and one reactor. The reactor is modelled in order to

perform the saponification reaction between the sodium hydroxide and ethyl acetate.

Saponification reaction of ethyl acetate and sodium hydroxide produced sodium acetate in a

batch and the continuous stirred tank reactor evaluate the rate data needed to design a

production scale reactor.

THEORY

Reaction Kinetics

Reaction kinetics is the branch of chemistry that quantifies rates of reaction. An elementary chemical reaction is a chemical reaction whose rate corresponds to a stoichiometric equation. In symbols:

A + B → C + D

and the reaction rate will be defined as:-r = k · (cA) α · (cB)β

where k is referred as the specific reaction rate (constant). The overall order of reaction is defined as:

n = α +β

The Mass Balance

Mass is a conservative entity, hence given a control volume V the sum of mass flows entering the system will equal the sum exiting minus (plus) the consumed (generated) or accumulated fractions:

Rate of mass in- rate of mass out+ rate of mass generated- rate of mass consumed = rate of mass accumulation

shortly:

IN – OUT + PROD – CONS = ACC

Residence Time

The reactor’s residence time is defined as the reactor volume divided by the total feed flow

rates.

Residence time, τ =

Conversion

The conversion (or fractional conversion), denoted X, is a frequently used measure of the

degree of reaction. It i s defined as

X =

EXPERIMENTAL PROCEDURES

General Start-Up Procedures

1) The following solution is prepared:

a) 40 L of sodium hydroxide, NaOH (0.1 M)

b) 40 L of ethyl acetate, Et(Ac) (0.1M)

c) 1 L of hydrochloric acid, HCl (0.25M), for quenching

2) Ensure that all valves are initially closed

3) The feed vessels are charged as follow.

a) The charge port caps for vessels B1 and B2 is opened.

b) The NaOH is carefully poured into vessel B1 and Et(Ac) into B2.

c) The charge port for both vessels is closed.

4) The power for the control panel is turned on.

5) Check that there is sufficient water in thermostat T1 tank. Refilled is necessary.

6) Cooling water valves, V13 is opened and let the cooling water flow through the

condenser W1.

7) Adjust the overflow tube to give a working volume of 10 L in reactor.

8) Valves V2, V3, V7, V8 and V11 were opened.

9) The unit was ready to run the experiment.

General Shut-Down Procedures

1) Keep the cooling water valve V13 open to allow the cooling water to continue

flowing

2) Pumps P1 and P2 is switched off. Stirrer M1 is switch off.

3) The thermostat T1is switched off. Let the liquid in the reaction vessel R1 cool down

to room temperature.

4) Cooling water valve V13 is closed.

5) Valves V2, V3, V7, and V8 were closed. Valves V4, V9 and V12 to drain the liquid

from unit.

6) The power for the control panel is turned off.

Preparation of calibration curve for conversion versus conductivity

1) The following solutions is prepared

a) 1L of sodium hydroxide, NaOH (0.1 M)

b) 1L of sodium acetate, Et(Ac) (0.1M)

c) 1L of deionised water, H2O

2) Determine the conductivity and NaOH concentration for each conversion values by

mixing the following solutions into 100 mL of deionised water.

a) 0% conversion : 100 mL NaOH

b) 25% conversion : 75 mL NaOH + 25 mL Et(Ac)

c) 50% conversion : 50 mL NaOH + 50 mL Et(Ac)

d) 75% conversion : 25 mL NaOH + 75 mL Et(Ac)

e) 100% conversion : 100 mL Et(Ac)

Back Titration Procedures for manual Conversion Determination

1) A burette is filled up with 0.1 M NaOH solution.

2) 10 mL of 0.25 M HCl is measured in a flask

3) 50 mL sample is obtained from the experiment and immediately add the sample to the

HCl in the flask to quench the saponification reaction.

4) 3 drops of phenolphthalein is added into the mixture.

5) The mixture is titrated with NaOH solution from the burette until the mixture is

neutralized. The amount of NaOH is recorded.

Effect of Residence Time of The Reaction in a CSTR

1) Perform the general start-up procedures.

2) Switch on both pumps P1 and P2 simultaneously and open valves V5 and V10 to

obtain the highest possible flow rate into the reactor.

3) Let the reactor fill up with both the solution until it is just about to overflow.

4) Readjust the valves V5 to V10 to give a flow rate of about 0.1 L/min. Make sure that

both flow rates are the same. Record the flow rate.

5) Switch on the stirrer M1 and set the speed to about 200 rpm.

6) Start monitoring the conductivity value at Q1-401 until it does not change over time.

This is to ensure that the reactor has reached steady state.

7) Record the steady state conductivity value and find the concentration of NaOH in the

reactor and extent of conversion from the calibration curve.

8) Open sampling valve V12 and collect 50 mL sample. Carry out a back titration

procedure to manually determine the concentration of NaOH in the reactor and extent

of conversion.

APPARATUS

1) Continuous stirrer tank reactor Model: BP 143

2) 50 mL burette

3) 200 mL beaker

4) Conical flask

5) Conductivity probe

6) Solution :

a) Sodium hydroxide (NaOH) 0.1M

b) Ethyl Acetate (Et(Ac)) 0.1 M

c) Deionised water

d) Phenolphthalein

7) 100 mL Measuring cylinder

RESULTS

Reactor volume = 10 L

Concentration of NaOH in feed vessel = 0.1 M

Concentration of Et(Ac) in feed vessel = 0.1 M

Temp

.

C)(ͦ

Flow rate

Of NaOH

(ml/min)

Flow rate

Of Et(Ac)

(ml/min)

Vol. of

NaOH

Titrated

(mL)

Total flow rate of

sol. F0

(ml/min)

Residence time,

(min)

Conduc-tivity

(mS/cm)

Exit conc. NaOH,

CNaOH,

(M)

Vol. of unreacted quenching HCl (mL)V2

Vol of HCl reacted with NaOH in sample (ml),V3

28.9 100 100 23.1 200 50 5.50 0.0038 9.24 0.76

28.9 150 150 26.0 30033

5.09 -0.0020 10.40 -0.40

28.9 200 200 25.7 40025

5.05 -0.0014 10.28 -0.28

28.9 250 250 28.2 50020

5.03 -0.0064 9.28 -1.28

28.9 300 300 30.6 60017

4.92 -0.0112 12.24 -2.24

Preparation of Calibration Curve

Conversion Solution Mixture Concentration

Of NaOH (M)

Conductivity

(mS/cm)0.1 M NaOH 0.1 M Et(Ac) H2O

0% 100 mL - 100 mL 0.0500 12.83

25% 75 mL 25 mL 100 mL 0.0375 7.29

50% 50 mL 50 mL 100 mL 0.0250 3.42

75% 25 mL 75 mL 100 mL 0.0125 5.75

100% - 100 mL 100 mL 0.0000 16.09

Rate Constant, k

k (M-1s-1) ln k 1/T (ºC-1)

133.24 4.89 0.035

772.73 6.65 0.035

20.44x103 9.93 0.035

129.88 4.87 0.035

52.15 3.95 0.035

No Rate Constant, k (M-1s-1) Reaction rate, -rA (M/s)

1 133.24 1.92 x 10-3

2 772.73 3.10 x 10-3

3 20.44x103 0.04

4 129.88 5.32

5 52.15 6.54 x 10-3

Graph of Calibration Curve

SAMPLE CALCULATIONS

NaOH + HCl → NaCl + H2O

Unknown quantity:

Concentration of NaOH in the reactor = CNaOH mol/L

Known quantities:

Volume of sample = Vs = 50 mL

Concentration of NaOH in the feed vessel = CNaOH,f = 0.1 mol/L

Volume of HCl for quenching = VHCl,s = 10 mL

Concentration of HCl in standard solution = CHCl,s = 0.25 mol/L

Volume of titrated NaOH = V1 mL

Concentration of NaOH used for titration = CNaOH,s = 0.1 mol/L

Calculations:

Conc. of NaOH entering the reactor, CNaOH,0 = ½ CNaOH,f mol/L

Vol. of unreacted quenching HCl, V2 = x V1 mL

Vol. of HCl reacted with NaOH in sample, V3 = VHCl,s - V2 mL

Moles of HCl reacted with NaOH in sample, n1 = (CHCl,s x V3)/1000 mol

Moles of unreacted NaOH in sample, n2 = n1 mol

Conc. of unreacted NaOH in the reactor, CNaOH = n2/ Vs x 1000 mol/L

Conversion of NaOH in the reactor, X = (1- ) x 100%

Residence time, τ =

τ = 10 L/(0.1+0.1) Lmin-1

= 50 min

Exit concentration of NaOH

CNaOH,0 = ½ CNaOH,f = 0.05 mol/L

V2 = x V1 = 0.1/0.25 x 23.1 = 9.24 mL

V3 = VHCl,s - V2 = 10 - 9.24 = 0.76 mL

n1 = (CHCl,s x V3)/1000 = 0.25 x 0.76/1000 = 1.9x10-4

n2 = n1 = 1.9x10-4 mol

CNaOH = n2/ Vs x 1000 = 1.9x10-4/50x1000 = 0.0038 M

X = (1- ) x 100% = [1-(3.8x10-3/0.05)]x100%

= 92.4%

Rate constant

k =

k1 = (0.1-0.0038)/50(0.0038)2

= 133.24 M-1s-1

Reaction rate

-rA = kCA2

k= 133.24

-rA = kCA2

= (133.24)(0.0038)2

= 1.92 x 10-3 M/s