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 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 reactor and the the extent extent of conversio conversion n from calibration calibration curve. curve. The 50ml 50ml sample 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 continuous stirred-tank reactor (CSTR) which is also known as vat- or back-mix back-mix reactor usually is a common ideal reactor type reactor type in chemical engineering. 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 slurries.. The behaviour of a CSTR is always modelled by that of a Continuous Ideally Stirred-Tank Reacto Reactorr (CISTR (CISTR). ). All calculat calculations ions performe performed d with with CISTR CISTRss assume assume perfect mixing mixing.. I n a perfectly mixed reactor, the output output composition composition is identical to composition 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 liquid phase phase under under adiabat adiabatic ic and isotherma isothermall conditi conditions ons.. CSTR CSTR consis consists ts of two tanks of solutio solutions ns and one reactor. reactor. The reactor reactor is modelle modelled d in order order to perform perform the saponi saponifica fication tion reaction reaction betwee between n the sodium sodium hydrox hydroxide ide and ethyl ethyl acetate acetate.. Saponi Saponifica fication tion reactio reaction n of ethyl 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 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 referred as the specific specific reaction reaction rate (const (constant). ant). The overall overall order order of reactio reaction n 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 sod sodium ium hyd hydrox roxide ide,, NaOH NaOH (0.1 (0.1 M) M) b) 40 L of of ethyl acetate, Et(Ac) Et(Ac) (0.1M) (0.1M) c) 1 L of hydroc hydrochlor hloric ic acid, acid, HCl (0.25 (0.25M), M), for for quench quenching ing 2) Ensure Ensure that that all all valves valves are initi initially ally clos closed ed 3) The feed vessel vesselss are charg charged ed as as follow follow.. a) The charg chargee port caps caps for vess vessels els B1 and and B2 B2 is opened opened.. b) The NaOH NaOH is carefully carefully poured poured into vessel B1 and Et(Ac) into B2. B2. c) The charge charge port port for for both both vess vessels els is closed. closed. 4) The powe powerr for the the contro controll panel panel is turne turned d on. 5) Check that there there is sufficient sufficient water water in thermostat thermostat T1 T1 tank. tank. Refilled is necessary. necessary. 6) Cool Cooling ing water water valv valves es,, V13 V13 is opened opened and let the the cool cooling ing water water flow flow throu through gh the condenser W1. 7) Adjust Adjust the overflow overflow tube to give a working working volume volume of 10 10 L in reactor. reactor. 8) Valves Valves V2, V2, V3, V3, V7, V8 V8 and V11 V11 were were opened opened.. 9) The unit unit was was ready ready to run the the experi experimen ment. t.
General Shut-Down Procedures
1) Keep the the cooling water valve valve V13 open open to allow allow the cooling cooling water water to continue continue flowing flowing 2) Pumps Pumps P1 and P2 P2 is switche switched d off. Stirrer M1 M1 is switch off. 3) The thermostat thermostat T1is T1is switched switched off. Let Let the liquid in the the reaction vessel vessel R1 cool cool down to room temperature. 4) Cooling Cooling water water valve valve V13 V13 is closed closed.. 5) Valves Valves V2, V2, V3, V7, and V8 V8 were closed closed.. Valves Valves V4, V9 and V12 V12 to drain the liquid liquid from unit. 6) The powe powerr for the the contro controll panel panel is turne turned d off.
Preparation of calibration curve for conversion versus conductivity
1) The The follow following ing solu solution tionss is prepa prepared red a) 1L of of sodium sodium hydrox hydroxide ide,, NaOH NaOH (0.1 (0.1 M) M) b) 1L of of sodium sodium acetate, acetate, Et(Ac) Et(Ac) (0.1M) (0.1M) c) 1L of deionised water, H 2O
2) Determ Determine ine the condu conductiv ctivity ity and NaOH concent concentratio ration n for each conversio conversion n values values by mixing the following solutions into 100 mL of deionised water. a) 0% conv convers ersion ion
: 100 100 mL NaOH NaOH
b) 25% conversion conversion : 75 mL mL NaOH NaOH + 25 mL Et(Ac) Et(Ac) c) 50% 50% conver conversion sion : 50 50 mL NaOH NaOH + 50 mL Et(Ac) Et(Ac) d) 75% conver conversion sion : 25 25 mL NaOH NaOH + 75 mL Et(Ac) Et(Ac) e) 100% 100% conver conversio sion n : 100 100 mL Et(Ac) Et(Ac)
Back Titration Procedures for manual Conversion Determination
1) A burette burette is filled filled up up with with 0.1 M NaOH NaOH solu solutio tion. n. 2) 10 mL mL of 0.25 0.25 M HCl HCl is measur measured ed in a flask flask 3) 50 mL sample sample is obtained obtained from the experim experiment ent and immedi immediatel ately y add the sample to the HCl in the flask to quench the saponification reaction. 4) 3 drops drops of of phenolphth phenolphthalein alein is added into the the mixture. mixture. 5) The The mixt mixtur uree is titrate titrated d with with NaOH solu solutio tion n from from the burett burettee until until the mixture mixture is neutralized. The amount of NaOH is recorded.
Effect of Residence Time of The Reaction in a CSTR
1) Perform Perform the the genera generall start-up start-up proc procedu edures. res. 2) Switch on on both pumps pumps P1 and P2 simultane simultaneously ously and open valves valves V5 and V10 to obtain obtain the highest possible flow rate into the reactor. 3) Let the reactor reactor fill up with with both both the solution solution until until it is just about about to overflow overflow.. 4) Readju Readjust st the valves valves V5 to V10 V10 to give a flow rate rate of about about 0.1 L/min. L/min. Make Make sure that that both flow flow rates rates are the same. Record Record the the flow rate. 5) Switch on the stirrer M1 M1 and set the speed speed to about 200 rpm. rpm. 6) Start Start monitorin monitoring g the conductiv conductivity ity value value at Q1-40 Q1-401 1 until it does does not change change over time. This is to ensure that the reactor has reached steady state. 7) Record Record the steady steady state conduc conductivi tivity ty value value and find the concentra concentration tion of NaOH NaOH in the reactor and extent of conversion from the calibration curve. 8) Open Open samp sampli ling ng valv valvee V12 V12 and and coll collec ectt 50 mL samp sample le.. Carr Carry y out out a back back titr titrat atio ion n procedure procedure to manually determine the concentration concentration of NaOH in the reactor and extent of conversion.
APPARATUS
1) Continu Continuous ous stirre stirrerr tank reacto reactorr Model: Model: BP 143 143 2) 50 mL burett rettee 3) 200 200 mL mL beak beaker er 4) Conic onical al flas flask k 5) Cond Conduc uctiv tivity ity probe probe 6) Solution ion : a) Sodi Sodium um hydr hydrox oxide ide (NaO (NaOH) H) 0.1M 0.1M b) Ethyl Acetate (Et(Ac)) (Et(Ac)) 0.1 0.1 M c) Deioni ionissed wat water er d) Phen Phenol olph phth thal alei ein n 7) 100 100 mL Mea Measu surin ring g cyli cylinde nder r
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.
Flow rate
Flow rate Vol. of
C (ͦ )
Of NaOH
Of Et(Ac)
NaOH
(ml/min)
(ml/min)
Titrated
Total flow rate of sol. F0 (ml/min)
(mL)
Residence time,
Conductivity
(min)
(mS/cm)
Exit conc. NaOH, CNaOH,
Vol. of Vol of unreacted reacted quenching with N HCl in sam (mL)V2 (ml),V3
(M)
28.9
100
100
23.1
200
28.9
150
150
26.0
300
28.9
200
200
25.7
400
28.9
250
250
28.2
500
28.9
300
300
30.6
600
50 33 25 20 17
5.50
0.0038
9.24
0.7
5.09
-0.0020
10.40
-0.
5.05
-0.0014
10.28
-0.
5.03
-0.0064
9.28
-1.
4.92
-0.0112
12.24
-2.
Preparation of Calibration Curve
Conversion
Solution Mixture H2O
Concentration
Conductivity
Of NaOH (M)
(mS/cm)
0.1 M NaOH
0.1 M Et(Ac)
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
=
C NaOH
mol/L
Known quantities: Volume of sample
=
Vs
=
50 mL
Concentration of NaOH in the feed vessel
=
C NaOH,f NaOH,f
=
0.1 mol/L
Volume of HCl for quenching
=
VHCl,s
=
Concentration of HCl in standard solution
=
CHCl,s
Volume of titrated NaOH
=
V1
Concentration of NaOH used for titration
=
C NaOH,s NaOH,s
10 mL =
0.25 mol/L mL
=
0.1 mol/L
Calculations: Conc. of NaOH entering the reactor, C NaOH,0 = NaOH,0 Vol. of unreacted quenching HCl, V 2
½ C NaOH,f NaOH,f
=
mol/L
x V1
mL
Vol. of HCl reacted with NaOH in sample, V 3
=
VHCl,s - V2
mL
Moles of HCl reacted with NaOH in sample, n 1
=
(CHCl,s x V3)/1000
mol
Moles of unreacted NaOH in sample, n 2
=
n1
mol
Conc. of unreacted NaOH in the reactor, C NaOH
=
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 C NaOH,0 NaOH,0 =
½ C NaOH,f NaOH,f
V2
=
V3
=
n1
=
= x V1
0.05 mol/L =
0.1/0.25 x 23.1 =
9.24 mL
VHCl,s - V2
=
10 - 9.24
=
0.76 mL
(CHCl,s x V3)/1000
=
0.25 x 0.76/1000
=
1.9x10 -4
n2
=
n1
=
1.9x10-4 mol
C NaOH
=
n2/ Vs x 1000
=
1.9x10 -4/50x1000
X
=
(1-
=
[1-(3.8x10-3/0.05)]x100%
=
92.4%
) x 100%
Rate constant
k
=
k 1
=
(0.1-0.0038)/50(0.0038) 2
=
133.24 M-1s-1
Reaction rate -r A
=
kCA2
k= 133.24 -r A
=
kCA2
=
(133.24)(0.0038) 2
=
1.92 x 10-3 M/s
=
0.0038 M
