PERTEMUAN 1
CHEMICAL REACTION ENGINEERING 1
RPKPS
Main reference:Fogler, H.S., 2006, Elements of Chemical Reaction
Engineering, 4th Ed., Pearson Education, Inc., Prentice Hall
Professional Technical Reference, New Jersey.
Community:elisa.ugm.ac.idTeknik Reaksi Kimia 1_TK_Budhijanto
1. MOLE BALANCESObjectives:After completing Chapter 1, you will
be able to: Define the rate of chemical reaction. Apply the mole
balance equations to a batch reactor, CSTR, and PFR. Describe
photos of real reactors.
Chemical kinetics:the study of chemical reaction rates and
reaction mechanisms.
Reactor:an equipment in which reactions occur
Chemical Reaction Engineering (CRE):combines the study of
chemical kinetics with the reactors.
How is a chemical engineer different from other engineers?It is
primarily a knowledge of chemical kinetics and reactor design that
distinguishes the chemical engineer from other engineers.
Why do we have to study CRE?The selection of a reaction system
that operates in the safest and most efficient manner can be the
key to the economic success or failure of a chemical plant. For
example, if a reaction system produces a large amount of
undesirable product, subsequent purification and separation of the
desired product could make the entire process economically
unfeasibleSpecific example: production of maleic anhydride (raw
material for various industries, e.g. polyester resins, paint,
etc)Main reaction:
Side reactions:
Reaction condition: 2 5 bar, 400 - 450C, vanadium catalyst
Chemical Identity:A chemical species is said to have reacted
when it has lost its chemical identity. The identity of a chemical
species is determined by the kind, number, and configuration of
that species' atoms. Three ways a chemical species can lose its
chemical identity: 1. Decomposition
2. Combination
3. Isomerization
Reaction Rate:the rate at which a species looses its chemical
identity per unit volume. It can be expressed as the rate of
disappearance of a reactant or as the rate of appearance of a
product. Consider species A:A BrA = the rate of formation of
species A per unit volumerA = the rate of a disappearance of
species A per unit volumerB = the rate of formation of species B
per unit volume If B is being created at a rate of 0.2 moles per
decimeter cubed per second, ie, the rate of formation of B is,rB =
0.2 mole/(dm3s)Then A is disappearing at the same rate:rA = 0.2
mole/(dm3s) The rate of formation of A isrA = 0.2 mole/(dm3s) For a
catalytic reaction, we refer to rA, which is the rate of
disappearance of species A on a per mass of catalyst basis. NOTE:
dCA/dt is not the rate of reaction
Example: Is sodium hydroxide reacting? Sodium hydroxide and
ethyl acetate are continuously fed to a rapidly stirred tank in
which they react to form sodium acetate and ethanol: NaOH +
CH3COOC2H5 CH3COONa + C2H5OH
The product stream, containing sodium acetate and ethanol,
together with the unreacted sodium hydroxide and ethyl acetate, is
continuously withdrawn from the tank at a rate equal to the total
feed rate. The contents of the tank in which this reaction is
taking place may be considered to be perfectly mixed. Because the
system is operated at steady state, if we were to withdraw liquid
samples at some location in the tank at various times and analyze
them chemically, we would find that the concentrations of the
individual species in the different samples were identical. Because
the species concentrations are constant and therefore do not change
with time,
where A is NaOH. If we defined
then
which is incorrect because C2H5OH and CH3COONa are being formed
from NaOH and CH3COOC2H5 at a finite rate. Consequently, the rate
of reaction as defined by
cannot apply to a flow system and is incorrrect if it is defined
in this manner. rA is an algebraic law!
is simply a mole balance that is only valid for a constant
volume batch system.Consider species j: rj is the rate of formation
of species j per unit volume [e.g. mol/(dm3s)] rj is a function of
concentration, temperature, pressure, and the type of catalyst (if
any) rj is independent of the type of reaction system (batch, plug
flow, etc.) rj is an algebraic equation, not a differential
equation We use an algebraic equation to relate the rate of
reaction, rA, to the concentration of reacting species (e.g. CA)
and to the temperature (T) at which the reaction occurs. Example:A
product
Which one is the correct one?It must be determined from
experimental observation.
Problem 1.1.: The Convention for Rates of Reaction Consider the
reaction A + 2B 3Cin which the rate of disappearance of A is 5
moles of A per dm3 per second at the start of the reaction.At the
start of the reaction (a) What is rA? (b) What is the rate of
formation of B? (c) What is the rate of formation of C? (d) What is
the rate of disappearance of C? (e) What is the rate of formation
of A, rA? (f) What is rB?
General Mole Balance Equation
A mole balance on species j at any instant in time, t:
WhereNj represents the number of moles of species j in the
system at time t.V is the reaction volume.
Batch Reactors
Assumption: the reaction is perfectly mixed.
Or
There are two types of batch reactor for gas phase reactions.1.
Constant volume batch reactors
2. Constant pressure batch reactors
Continuous Stirred Tank Reactor (CSTR = Vat = Backmix Reactor)
Assumption: the reaction is perfectly mixed.Thus:1. 2. The
conditions in the exit stream (e.g. concentration, temperature) are
identical to those in the tank.
At steady state:
We get:
Or
Because:
Wherev = the volumetric flow rate (volume/time)
Tubular Reactor Assumption: plug flow no radial variation in
reaction rate the reactor is referred to as a Plug Flow Reactor
(PFR).
Fj0Fj,exitVVFjV)FjV+VThe differential volume, V, is sufficiently
small such that there are no spatial variations in reaction rate
within its volume.At steady state:
limit V0:
Or
Summary:General Mole Balance Equation:
ReactorMole BalanceRemark
Batch
Perfectly mixed
CSTR
Perfectly mixed; Steady State
PFR
Steady State
Akhir Kuliah 1Problem 1.2.: How Large Is It? Consider the liquid
phase cis-trans isomerization of 2-butene.
which we will write symbolically asA BThe first order ( reaction
is carried out isothermally in a tubular reactor in which the
volumetric flow rate, v, is constant = 10 L/min. The exiting
concentration of A is 10% of its entering concentration. k =
0,23/min.a. Sketch the concentration profile inside the PFRb.
Determine the reactor volume
Problem 1.3.:The same as Problem 2b, but instead of using a PFR,
the reaction occured in an isothermal CSTR.
Problem 1.4.:A 200 L constant-volume batch reactor is
pressurized to 20 atm with a mixture of 75% A and 25% inert. The
gas-phase reaction is carried out isothermally at 227 C.
V = 200 LP = 20 atmT = 227C a. Assuming that the ideal gas law
is valid, how many moles of A are in the reactor initially? What is
the initial concentration of A? b. If the reaction is first
order:
Calculate the time necessary to consume 99% of A. c. If the
reaction is second order:
Calculate the time to consume 80% of A. Also calculate the
pressure in the reactor at this time if the temperature is 127C.
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