ABSTRACT
This is the experiment of continuous stirred tank reactor 40 L. The reaction to be studied is the saponi saponific ficati ation on reacti reaction on of ethyl ethyl acetat acetatee Et(Ac) Et(Ac) and sodium sodium hydroxi hydroxide de Na! Na! in the continuous stirred tank reactor. The calibration cur"e of con"ersion "s conducti"ity is also perform in this experiment. The ob#ecti"es of this experiment are to perform saponification reaction bet$een Na! and Et(Ac)% to determine the reaction rate constant and to study the effect of residence time on the con"ersion. This experiment is done by "aryin& the residence time by manipulatin& the flo$ rate of the system. The flo$ rate of both feeds that is introduced% Et(Ac) and Na! is the same and "aried from 0.' Lmin to 0. mLmin. *rom the result% the percenta&e of con"ersion of Na! is increasin& $ith the increase of residence time. +hen the residence is ,00 mins% the con"ersion is -.4/. Then as the total flo$ rate pro"ided for the system is increased% the residence time is decreasin& and that makes the con"ersion of Na! to decrease as $ell. +hen the residence time is .1 mins% the con"ersion is increasin& to -.4/. The lo$er flo$ rate rate &i"es a hi&her residence time time and helps increasin& the con"ersion of the reaction in the 23T.
1
INTRODUCTION
2hemical kinetics and reactor desi&n are at the heart of producin& almost all industrial chemicals. The selection of a reaction system that operates in the safest and most efficient manner can be the key to the success or failure of a chemical plant. The reaction occurr occurred ed in a reacto reactorr is exotherm exothermic ic or endothe endothermi rmic. c. A chemica chemicall reacto reactorr is a "essel "essel $here reactions are carried out purposefully to produce products from reactants by means of one or more more chemica chemicall reacti reactions ons.. A chemical chemical reacto reactorr may be charac character teri5e i5ed d by the mode mode operat operation ion accordin& to the flo$ condition. 2ontinuous stirred tank reactors (23T) are the most basic of the continuous reactors used in chemical processes. The continuous stirred tank reactor or back mix reactor is a "ery common processin& unit in chemical and polymer industry. 6ts names su&&est% it is a reactor in $hich the contents are $ell stirred and uniform throu&hout. The 23T is normally run at steady state% and is usually operated so as a to be 7uite $ell mixed. The 23T is &enerally modele modeled d as ha"in& ha"in& no spatia spatiall "ariat "ariation ionss in concent concentrat ration ions% s% temper temperatu ature% re% or reacti reaction on rate rate throu&hout the "essel. 3ince the temperature and concentration are identical e"ery$here $ithin the reaction "essel% they are the same sa me at the exits point as they are else$here in the tank. 2ontinuous stirred‐tank reactors (23Ts) are open systems% $here material is free to enter or exit the system that operate on a steady‐state basis% $here the conditions in the reactor don8t chan&e $ith time. eactants are continuously introduced into the reactor% $hile products are continuously remo"ed. 23Ts are "ery $ell mixed% so the contents ha"e relati"ely uniform properties such as temperature% density% etc. throu&hout. Also% conditions in the reactor8s exit stre stream am are are the the same same as thos thosee insi inside de the the tank. tank. 2ont 2ontin inuo uous us stir stirre red d‐tank tank reacto reactors rs are most most commonly used in industrial processin&% primarily in homo&eneous li7uid‐ phase flo$ reactions% $here constant a&itation is re7uired. They may be used by themsel"es% in series% or in a parallel. p arallel. The 2ontinuous 3tirred Tank eactor (9odel: ;< '4) unit is suitable for student experiments on continuous chemical reactions. The unit consists of a #acketed reaction fitted in the a&itator and condenser. The unit comes complete $ith "essels for ra$ materials and product% feed pumps% and thermostat. . 3tudents $ill conduct continuous saponification reaction of ethyl 2
acetate and sodium hydroxide. A conducti"ity measurement is pro"ided and students $ill relate the conducti"ity "alue to extent of reaction. 3tudents $ill study the effects of residence time and reaction temperature on the reaction rate constant.
OBJECTIVE:
To carry out a saponification reaction bet$een Na! and Et(Ac) in a 23T To determine the effect of residence time onto the reaction extent of con"ersion To determine the reaction rate constant
3
THEORY
The beha"ior of a 23T is often approximated or modeled by that of a 2ontinuous 6deally 3tirredTank eactor (263T). All calculations performed $ith 263Ts assume perfect mixin&. 6n a perfectly mixed reactor% the output composition is identical to composition of the material inside the reactor% $hich is a function of residence time and rate of reaction. 6f the residence time is = to '0 times the mixin& time% this approximation is "alid for en&ineerin& purposes. The 263T model is often used to simplify en&ineerin& calculations and can be used to describe research reactors. 6n practice it can only be approached% in particular in industrial si5e reactors. Assume: •
6nte&ral mass balance on number of moles Ni of species i in a reactor of "olume >?
$here *io is the molar flo$ rate inlet of species i% *i the molar flo$ rate outlet% and stoichiometric coefficient. The reaction rate% r% is &enerally dependent on the reactant concentration and the rate constant (k). The rate constant can be determined by usin& a kno$n empirical reaction rates that is ad#usted for temperature usin& the Arrhenius temperature dependence. @enerally% as the temperature increases so does the rate at $hich the reaction occurs. esidence time% is the a"era&e amount of time a discrete 7uantity of rea&ent spends inside the tank. Assume: •
constant density ("alid for most li7uids? "alid for &ases only if there is no net chan&e in
•
the number of moles or drastic temperature chan&e) isothermal conditions% or constant temperature (k is constant) steady state
•
4
sin&le% irre"ersible reaction (A B ') first order reaction (r B k2A)
• •
A C products NA B 2A > ($here 2A is the concentration of species A% > is the "olume of the reactor% NA is the number of moles of species A)
The "alues of the "ariables% outlet concentration and residence time% in the e7uation are ma#or desi&n criteria. To model systems that do not obey the assumptions of constant temperature and a sin&le reaction% additional dependent "ariables must be considered. 6f the system is considered to be in unsteady state% a differential e7uation or a system of coupled differential e7uations must be sol"ed. 23T8s are kno$n to be one of the systems $hich exhibit complex beha "ior such as steady state multiplicity% limit cycles and chaos. 2on"ersion: ne of the reactants is choosed as the basis of calculation and the other species in"ol"ed is related in the reaction to this basis. 2onsider the &eneral e7uation aA D b; C c2 D d +e $ill choose A as our basis of calculation.
AD
b a
B C
c d C + D a a
The basis of calculation is most al$ays the limitin& reactant. The con"ersion of species A in a reaction is e7ual to the number of moles of A reacted per mole of A fed.
5
APPARATUS/MATERIALS
• • • •
tubular flo$ reactor deioni5ed $ater sodium hydroxide ethyl acetate
6
PROCEDURE/METHODOLOGY
General Start-Up Pr!e"#re$
'. The follo$in& solutions are prepared: a) 40 L of sodium hydroxide% Na! (0.'9) b) 40 L of ethyl acetate% Et(Ac) (0.'9) c) ' L of hydrochloric acid% !2L (0.,=9)% for 7uenchin& ,. All "al"es are ensured to be initially closed. . The feed "essels are char&ed as follo$s: a) The char&e port caps are opened for "essels ;' and ;,. b) The Na! solution is carefully poured into "essel ;' and the Et(Ac) solution into "essel ;,. c) The char&e port caps for both "essels are closed. 4. The po$er for the control panel is turned on. =. +ater in the thermostat T' tank is check $hether it is sufficient. efill as necessary. . 2oolin& $ater "al"e >' is opened and the coolin& $ater is let to flo$ throu&h the condenser +'. (nly for Experiment ,) 1. The o"erflo$ tube is ad#usted to &i"e a $orkin& "olume of '0 L in the reactor '. . >al"es >,% >% >1% > and >'' are opened. -. The unit is no$ ready for experiment.
7
General S%#t-D&n Pr!e"#re$
'. The coolin& $ater "al"e >' is kept open to allo$ the coolin& $ater to continue flo$in&. ,. ;oth pumps <' and <, are s$itched off. 3tirrer 9' is s$itched off. . The thermostat T' is s$itched off. The li7uid in the reaction "essel ' is let to cool do$n to room temperature. 4. 2oolin& $ater "al"e >' is closed. =. >al"es >,% >% >1 and > is closed. >al"es >4% >- and >', are opened to drain any li7uid from the unit. . The po$er for the control panel is turned off.
A' Preparat(n ) Cal(*rat(n C#r+e )r Cn+er$(n +$' Cn"#!t(+(t,
The reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxide Na!. 3ince this is a second order reaction% the rate of reaction depends on both concentrations of Et(Ac) and Na!. !o$e"er% for analysis purposes% the reaction $ill be carried out usin& e7uimolar feeds of Et(Ac) and Na! solutions $ith the same initial concentrations. This ensures that both concentrations are similar throu&hout the reaction. Na! D Et(Ac)
Na(Ac) D Et!
The follo$in& procedures $ill calibrate the conducti"ity measurements of con"ersion "alues for the reaction bet$een 0.' 9 ethyl acetate and 0.' 9 sodium hydroxide: Pr!e"#re$:
'. The follo$in& solutions is prepared: a) ' liter of sodium hydroxide% Na! (0.' 9) b) ' liter of sodium acetate% Na(Ac) (0.' 9)
8
c) ' liter of deionised $ater% !, ,. The conducti"ity and Na! concentration for each con"ersion "alues are determined by mixin& the follo$in& solutions into '00 ml of deionised $ater: a) 0/ con"ersion : '00 ml Na! b) ,=/ con"ersion : 1= ml Na! D ,= ml Na(Ac) c) =0/ con"ersion : =0 ml Na! D =0 ml Na(Ac) d) 1=/ con"ersion : ,= ml Na! D 1= ml Na(Ac) e) '00/ con"ersion : '00 ml Na(Ac)
B' Ba! T(trat(n Pr!e"#re$ )r Man#al Cn+er$(n Deter.(nat(n
6t is ad"isable to carry out manual con"ersion determination on experiment samples to "erify the conducti"ity measurement "alues. The follo$in& procedures $ill explain the method to carry out back titration on the samples. 6t is based on the principle of 7uenchin& the sample $ith excess acid to stop any further reactions% then back titratin& $ith a base to determine the amount of unreacted acid. Pr!e"#re$:
'. A burette is filled up $ith 0.' 9 Na! solution. ,. '0 ml of 0.,= 9 !2l is measured in a flask. . A =0 ml sample from the experiment is obtained and immediately added to the !2l in the flask to 7uench the saponification reaction. 4. A fe$ drops of p! indicator is added into the mixture. =. The mixture is titrated $ith Na! solution from the burette until the mixture is neutrali5ed. The amount of Na! titrated is recorded.
9
Eper(.ent pr!e"#re:
'. The &eneral startFup procedures is performed. ,. ;oth pumps <' and <, is s$itched on simultaneously and open "al"es >= and >'0 to obtain the hi&hest possible flo$ rate into the reactor. . The reactor is filled up $ith both of the solution until it is #ust about to o"erflo$. 4. The "al"es >= and >'0 are read#usted to &i"e a flo$ rate of about 0.' Lmin. ;oth flo$ rates are made sure to be the same. The flo$ rate is recorded. =. The stirrer 9' is s$itched on and the speed is set to about ,00 rpm. . The conducti"ity "alue at G6F40' is monitored until it does not chan&e o"er time. This is to ensure that the reactor has reached steady state. 1. The steady state conducti"ity "alue is recorded and the concentration of Na! in the reactor and extent of con"ersion is found from the calibration cur"e. . 3amplin& "al"e >', is opened and a =0 mL sample is collected. A back titration procedure to is determined manually determine the concentration of Na! in the reactor and extent of con"ersion. -. The experiment (steps = to -) is repeated for different residence times by ad#ustin& the feed flo$ rates of Na! and Et(Ac) to about 0.'=% 0.,0% 0.,= and 0.0 Lmin. ;oth flo$ rates are made sure to be the same.
10
RESULTS
Table of the preparation of calibration cur"e. Cn+er$(
Sl#t(n M(t#re$ 2'3M
n
NaOH
0/ ,=/ =0/ 1=/ '00/
'00mL 1=mL =0mL ,=mL F
2'3M
Cn!entrat(
Cn"#!t(+(t
H4O
n ) NaOH
,
'00mL '00mL '00mL '00mL '00mL
0M1 0.0=00 0.01= 0.0,=0 0.0',= 0.0000
0.S/ !.1 '0.1 -.1 1.= =. 4.0
Na0A!1
F ,=mL =0mL 1=mL '00mL
Table for experiments ' eactor "olume B 40 L 2oncentration of Na! in feed "essel B 0.' 9 2oncentration of Et (Ac) in feed "essel B 0.' 9
5l&
5l&
Ttal
Re$("en
Cn"
E(t
Cn+er$
Rea!t(n
Rea!t(n
Vl )
rate )
rate )
)l&
!e
#!t(+(t
Cn!ent
(n6
rate
rate
NaOH
NaOH
Et0A!1
rate )
t(.e6 t
,6 7
rat(n )
8 091
!n$tant6
T(trate
0L/.(n
0L/.(n
$l#t(
0.(n1
0.S/!
NaOH
"
1
1
n$6 V2
.1
0L1
0L / 5T 23 0.'0
5T 24 0.'0
.(n1 0.,
,00
.='
0.000
-.4
4.1=
,.4 x '0F4
0.0,4
0.'=
0.'=
0.
'.
.'1
0.00'0
-
1.=
. x '0F4
0.0,==
0.,0 0.,0 0.4 '00 0.,= 0.,= 0.= 0 0.0 0.0 0. .1 SAMPLE CALCULATIONS
,. ,.4 ,.=,
0.00,0 0.00' 0.00'
- -.4 -.4
',0 '=.-=1 ,,.'4
4. x '0F4 .0 x '0F4 1., x '0F4
0.0,0 0.0,=0.0,=-
Re$("en!e T(.e: 11
2alculation for flo$ rates of 0.' Lmin: Total flo$ rate% >o
B *lo$ rate of Na! D *lo$ rate of Et(Ac) B 0.' Lmin Na! D 0.' Lmin Et(Ac) B 0., Lmin
τ = esidence Time%
Reactor volume ( L ) ,V Total flow rate
( )
L , v0 min
=
40 L 0.2 L / min
B ,00 mins
In(t(al !n!entrat(n ) NaOH enter(n t%e rea!tr:
Concentration of NaOH ∈ feed B
2
0.1
B
2
B 0.0= 9
Cn+er$(n:
2alculation for flo$ rates of 0.' Lmin : 9oles of reacted Na!% n' n' B 2oncentration Na! x >olume of Na! titrated B 0.' 9 x 0.0,4. L B 0.00,4 mole 9oles of unreacted !2l% n, B 9oles of reacted Na!% n' n, B 0.00,4 mole
12
>olume of unreacted !2l% >'
n2 >'
B
concentration HCl quench 0.00246
B
0.25
B 0.00-4 L >olume of !2l reacted% >, >, B Total "olume !2l H >' B 0.0' H 0.00-4 B 0.000' L 9oles of reacted !2l% n n B 2oncentration !2l x >, B 0.,= x 0.000' B 0.00004 mole 9oles of unreacted Na!% n4 n4 B n B 0.00004 mole 2oncentration of unreacted Na!
n4 2 Na! unreacted
B
volume sample 0.00004
B
0.05
B 0.000 9
Iunreacted Iunreacted B
Concentration of NaOH unreacted concentration NaOH 0.0008
B
0.05
B 0.0' Ireacted Ireacted
B ' F Iunreacted B ' F 0.0' B 0.-4
2on"ersion for flo$ rate 0.' Lmin% 13
0.-4 x '00/ B -.4 / eaction rate constant% k k B ( 2Ao H 2A) J2A, B ( 0.0= H 0.000) (,00 x 0.000,) B 4.1= 9F' min F' ate of reaction% Fr A Fr A B k2A, B ,4.44 x 0.00, B .1- x '0 F4 molL.min
14
DISCUSSION
Conversion vs Conductivity 12 10 8
Conductivity (mS/cm)
f(x) = - 0.07x + 11 R² = 0.99
6 4 2 0 0 10 20 30 40 50 60 70 80 90 100
Conversion (%)
*or the preparation of calibration cur"e for con"ersion "s conducti"ity% the Na! solution and Na(Ac) is mixed toðer in order to &i"e a "alue of con"ersion from the 0/ $hich means a pure Na! solution to the '00/ con"ersion $hich means a pure Na(Ac) solution produced. The conducti"ity of the solution is then obtained by usin& a conducti"ity meter. The cur"e obtained is sho$n in the fi&ure abo"e. The hi&her the con"ersion of Na!% the lo$er the conducti"ity "alue obtained based on the result. The best fit lined constructed from the result &i"es the line of y B F0.01x D ''. Thus% the slope obtained is F0.01 $hile the yFintercept is at '' m3cm. The 23T is normally run at steady state% and is usually operated so as a to be 7uite $ell mixed. The conducti"ity "alue in the 23T is assumed as the same any$here in the reactor. *rom the experiment% the conducti"ity of the reactant decreases as the con"ersion decreases. The -.4/ con"ersion &i"es a conducti"ity "alue of .=' m3cm$. The con"ersion $hich is -.4/ &i"es the lo$est conducti"ity "alue of ,.=, m3cm. The decrease in the conducti"ity "alue of the solution is because of the decrease in the ionic acti"ity of a solution in term of its capacity to transmit current. As the electrical current is transported by the ions in solution the conducti"ity increases as the concentration of ions increases. Thus% the hi&her con"ersion of the Na! &i"es a 15
lo$er ionic acti"ity for the preparation of the calibration cur"e result. !o$e"er% the lo$er con"ersion of the Na! &i"es a lo$er ionic acti"ity from the experiment conducted. The ionic acti"ity is hi&her in the hi&her con"ersion of Na!. This is possibly because of the a&itation in 23T $hich makin& the ionic acti"ity still &oin& o n in the solution e"en after the con"ersion.
Conversion vs Residence Time 100 95
Conversion (%)
90 85 80 60
80
100
120
140
160
180
200
Residence Time (min)
*or the experiment of in"esti&atin& the effect of con"ersion on residence time% the reaction to be studied is the saponification reaction of ethyl acetate Et(Ac) and sodium hydroxide Na!. The reaction is carried out usin& e7uimolar feeds of Et(Ac) and Na! solutions $ith the same initial concentrations. This ensures that both concentrations are similar throu&hout the reaction. The introduced flo$ rate of both feeds also the same throu&hout the experiment. The flo$ rate is then "aried from 0.' Lmin to .0 Lmin for both feeds in order to &i"e "ariation in the residence time. The residence time is determined from the e7uation:
τ = esidence Time%
Reactor volume ( L ) ,V Total flowrate
( )
L , v0 min
The results obtained from the experiment are sho$n in the &raph abo"e. *rom the &raph% $e can see that the percenta&e of con"ersion of Na! is increasin& $ith the increase of
16
residence time. +hen the residence is ,00 mins% the con"ersion is -.4/ that is the hi&hest con"ersion of Na! from the experiment. Then as the total flo$ rate pro"ided for the system is increased% the residence time is decreased and that makes the con"ersion of Na! to decrease as $ell. +hen the residence time is .1 mins% the con"ersion is decreasin& to -.4/. The 23T is normally run at steady state% and is usually operated so as a to be 7uite $ell mixed. *rom the result% $hen flo$rate is lo$% it helps the reaction more in the 23T as con"ersion "alue is hi&her as sho$n in the experiment. This is because the reactants ha"e a lot of time to make contact $ith each other before exitin& the outlet $hen the flo$ rate is lo$. Thus% the lo$er flo$rate helps the con"ersion to increase as the contact bet$een the reactant. As for the reaction rate of the reaction% Fr A from the result it is increasin& as the residence time is hi&her $hile the rate constant% k is decreasin&.
17
CONCLUSION
As a conclusion% the ob#ecti"es for this experiment are achie"ed. This is because $e &et to perform saponification reaction bet$een Na! and Et(Ac) by usin& the continuous stirred tank reactor 40 liter unit and then determine the reaction rate constant for each reaction $hich is different from each flo$ rate introduced to the system. ;esides that $e also &et to study the effect of residence time on the con"ersion $hich is the lo$er flo$ rate &i"es a hi&her residence time and increasin& the con"ersion in the reaction.
RECOMMENDATION
'
6t is recommended that the same experiment is conducted by usin& other type of reactor in order to kno$ $hich reactor $ill &i"e a better process for the reaction and their characteristics can be compared.
,
6t is recommended that the flo$rate of the feed Na! and Et(Ac) to be "aried and not the same $ith each other to kno$ ho$ $ill it affects the reaction process in the continuous stirred tank reactor.
6t is also recommended to "aried the concentration of the feed for example like ho$ $ould the reaction process &o if the feed introduced into the system is not e7uimolar.
18
RE5ERENCES
'
Amrita >irtual Lab 2ollaborati"e
,
Encyclopedia f 2hemical En&ineerin& E7uipment. etrie"ed on th April ,0'= from http:encyclopedia.che.en&in.umich.edu
*o&ler% !.3 (,00). Elements of 2hemical eaction En&ineerin& (rd Edition).
4
Le"enspiel% . ('---). 2hemical eaction En&ineerin& (rd Edition). Mohn +iley.
=
+ikipedia. 2ontinuous 3tirred Tank eactor. etrie"ed on th April from http:en.$ikipedia.or&$iki2ontinuousstirredtankreactor
19
APPENDI8
20
