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CHEMICAL REACTORS

The mathematical models proposed for ideal plug-flow and continuous stirred-tank

reactors do not include the effects of the flow pattern on conversion. In a plug-flow

reactor the velocity profile is assumed to be flat and the dispersion of reactants andproducts by turbulence and diffusion is neglected. In the CSTR all reactants and products

are presumed to be perfectly mixed with the reactor contents as soon as they enter the

vessel. These reuirements for the ideali!ed models are usually not met in practice.

The backmixing created by turbulence and diffusion in a tubular reactor produces a

conversion which is lower than predicted by the plug-flow model. Since backmixing is

not as extensive as in a stirred tank" conversion in a tubular reactor is often higher thanconversion in a CSTR.

The deviation of an actual tubular reactor from the plug-flow model is a function of the

bulk velocity. In a laminar flow" where velocity profile is parabolic" the deviation islargest. #nder conditions of fully developed turbulence $Re %&"&&&' the velocity

profile is more flat and the plug-flow model is more closely approximated.

In this experiment" the deviation of experimental reactors from ideali!ed models will be

illustrated. The use of a batch reactor to generate data for the determination of kineticparameters will also be considered.

Theory

The reaction studied in this is the saponification of ethyl acetate. This reaction is

elementary and second-order. The reaction euation is

(a)* + ,t)c (a)c + ,t)*

+ --/ C + 0

(A) Batch Reactor

1or a constant volume isothermal batch reactor" the design euation is2

dC

dtr

A

A=

1or a bimolecular second order reaction" the rate euation is2

-r3 k CC


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In the case where eual amounts of and are charged to the reactor" the stoichiometry

table looks like this2

S4,CI,S I(ITI5 C*(6, 1I(5

(o -7(o (o$%-7'

(o3 (ao -7(o (o$%-7'C (Co3 & +7(o (o7

0 (0o3 & +7(o (o7

88888888 888888 88888888(To & (To3 (T

where 7 3 fractional conversion of

9 4lease note" this year $:&&%' the initial concentrations are ()T the same and you willhave to ad;ust the euations and your analyses accordingly.

The design and rate euations are written in terms of concentrations. Thus" we need to

use the relations2 C C X

C C X

A Ao

B Bo

=

=

$ '

$ '

%

%

If we combine the above euations" we see that

dC

dtr kC X

A

A Ao= = : :%$ '

utdC

dt C

dX

dt

A

Ao=

ThereforedX

dt kC XAo $ '%

:

If we integrate both sidesdX

XkC dt Ao

tx

$ '% : && =

we see that X

XktCAo

% =

$%'

Conductivity Measurements


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The conductivity probe produces a voltage which is related to the concentration of ions in

the solution.

V K Ct i ii

=

where2 3 proportionality constant

There are three ions present in this system2 )*-" C*?C))-" and (a+.

C C C X

C C C X

C C C C

OH B Ao

CH COO C Ao

Na B C Ao

+

= =

+ =

= + =

$ '%

?

Therefore" the total voltage read by the probe is

[ ]V K C X C X C t Ao OH Ao CH COO Ao Na= + + +$ '% ?

V KC X X t Ao OH CH COO Na= + +

+$ '%

?

t the start of the experiment" 7 3 & and let


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V V

V VX

C C

C

o t

o c

A

A

Ao

o

= =

$:'

If we substitute ,uation $:' into ,uation $%'" we get

V V

V VC kt

o t

t c

Ao

=

and

V V VC kt

Vt o tAo

c= +$ ' %

$?'

Therefore" a plot of


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E' 0rain the reactor contents and clean the vessel.

(B) CSTR#s

In this experiment the conversion in a CSTR system is compared to the conversion in a

batch reactor. Two %E&-liter tanks of prepared solution should be ready for you in thelaboratory. The concentration of (a)* is approximately &.&E( while ethylacetate is

approximately at &.&D:(. The reagents are to be reacted at eual volumetric flows.

"rocedure

%' Start 4umps" set rotameter reading at D& and record the conductivity as a function of

time in the :ndCSTR. Ahen steady state is reached" record the conductivity in eachof the CSTRFs.

:' Change the flow rate from D& to @& on the rotameter" record the change in

conductivity as a function of time in the :

nd

CSTR and determine steady-stateconductivities in the two reactors. )bserve volume changes in the reactors.

?' Repeat procedure :' for the flow rate of %& on the rotameters.

B' Record temperature of the solution before and after the reaction is completed.

(C) Tu$u!ar Reactor

Two %E&-litre tanks of prepared solution should be ready for you in the laboratory.

The approximate concentration of (a)* is &.&E( while ethyl acetate is &.&D:(. The

reagents are to be reacted at eual volumetric flows.

Exerimenta! "rocedure

%' )pen valves at the base of the supply tanks.

:' Turn on the pumps and ad;ust the flow rate so that the floats of both rotameters are

at @&.

?' fter :-? residence times record flow rate" conductivity and temperature.

Calibration curves for rotameters are available in the lab.

B' Repeat Step ? for a few other rotameter readings. 0o not wait too long between

runs or you may run out of reagent.

@' Record temperature of the solution at the outlet of the reactor.


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' Ahen tanks are empty" shut off pumps" close valves at the base of the tanks and fill

both tanks with tap water.

(%) O$servation o& '!o "attern in a Tu$u!ar Reactor

%' )pen valves at the base of the tanks and start pumps. Set flow rates at rotameter

reading of @&.

:' In;ect about %& m5 of concentrated >Gn)Bsolution at the entrance HTF of the reactor.

,stimate the length of the colored region at the entrance" middle" and exit of the

reactor.

?' Repeat step $:' for rotameter readings %& and %&& and ualitatively compare the

lengths of the colored region to the length in step $:'.

Reort

%' The report should include a comparison of the conversion in a batch reactor with the

conversion in the CSTRFs $or tubular reactor' for the same reaction time.

:' 0etermine the rate constants from results for the batch reactor and check to see if thisrate constant predicts the steady state conversion in the CSTRFs or tubular reactor.

?' 1ormulate a model for the system of two CSTRFs $or tubular reactor' which should

describe the change in concentration due to changes in inlet flow rate.

B' Compare the experimental transient response with model prediction. 0iscuss the

discrepancy" if any.

@' Comment on the differences in flow pattern that exist at various flowrates within the

tubular reactor. 0iscuss how these various flow patterns could affect the assumptionmade about a 41R.

Re&erences

%. 5evenspiel" )." Chemical Reaction ,ngineering" ohn Ailey J Sons" Toronto" ,d. :"

%KD:" p. :E?-K&.

:. >endall" *.." n pparatus for #ndergraduate ,xperiments in 1low System

Reaction >inetics" Chemical ,ngineering 4rogress Symposium Series" vol. D&" no.

?" p. ?-%.

?. *ovorka" R.." >endall" *.." Tubular Reactor at 5ow 1low Rates" Chemical

,ngineering 4rogress" vol. @" no. E" ug. %K&" p. @E-:.


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