Chemical Reaction Engineering

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Chemical Reaction Engineering
Lecture 8
Lecturer ???
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This course focuses on unsteady-state non-isother
mal reactors.
We have discussed the steady-state heat effects
on reaction engineering. What about
unsteady-state?
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Thermodynamics

• First law
• To a closed (mass) system

The work done by the system to the surroundings
The change in total energy of the system
The heat flow to the system
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An open system (for example, CSTR)
Rate of flow of heat to the system from the
surroundings
Rate of energy leaving the system by mass flow
out the system
Rate of accumulation of energy in the system
Rate of work done by the system on the
surroundings
Rate of energy added to the system by mass flow
into the system
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The work term
Rate of work done by the system on the
surroundings

• The work term is usually saparated into flow
  work and other work.
• Flow work
• the work that is necessary to get the mass into
  and out of the system
• for example, when shear stresses are absent

P pressure Vi specific volume
Flow work other work (shaft work)
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Such as electric or magnetic energy, light etc.
Potential energy
Kinetic energy
Internal energy
Usually
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Enthalpy!!, function of T
unit (cal / mole)
Unsteady state
the rate of the change of the total energy of
the system
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(No Transcript)
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CSTR no spatial variations in the system volume
and total pressure
CSTR
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No phase change
For liquid phase
For liquid phase
Cps is the heat capacity of the solution
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Batch no spatial variations in the system
volume, pressure, temperature and species
concentration
Batch
Batch
For adiabatic conditions, the relationship
between temperature and conversion is the same
for batch reactors, CSTRs, PBRs, and PFRs.
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Adiabatic batch reactor example
Propylene glycol is produced by the hydrolysis of
propylene oxide
An insulated instrumented 10-gal stirred batch
reactor is used. 1 gal of methanol and 5 gal of
water containing 0.1 wt H2SO4 are charged with
initial temperature 58F. How many minutes should
it take the mixture inside the reactor to reach a
conversion of 51.5 if the reaction rate law is
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Mass balance (batch)
Stoichiometry
Rate laws
Energy balance
Cps
Solved together
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We can also obtain T (t) and X (t) , page 541
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Another example for the exothermic reaction -
Safety
Production of nitroanaline is from the reaction
of ammonia with o-nitrochlorobenzene (ONCB)
Normally, the batch reaction is carried out
isothermally at 175 C and about 500 psi using
the cooling water with the ambient temperature 25
C. The operation condition was changed as V
5.119 m3 9.044 kmol of ONCB 33.0 kmol of NH3
103.7 kmol of H2O Normally was V 3.26 m3
3.17 kmol of ONCB 43.0 kmol of NH3 103.6 kmol
of H2O
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Mass balance (batch)
Stoichiometry
Rate laws
Energy balance
assuming 0
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Case (I) Isothermal up to 45 min (k constant)
From t 0 t 45 min
X 0.033
At this time (t 45 min)
safe
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Case (I) Adiabatic for 10 minutes (from 45 min
to 55 min)
Initial condition t 45 min X 0.033 T
448K
At time t 55 min X 0.0424 T 468 K
At time (t 55 min)
Return of the cooling system gives
The point of no return has been passed! The
temperature will continue to return, which leads
to explosion!
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When will the explosion occurs?
If the pressure relief disk work proporly at 265
C
safe
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Heat effects in a semibatch reactor (example)
Second-order saponification of ethyl acetate is
carried out in a semibatch reactor
Aqueous sodium hydroxide is fed at a
concentration of 1 kmol/m3, a temperature of 300
K, and a rate of 0.004 m3/s to 0.2 m3 of a
solution of water and ethyl acetate. The
initial concentrations of ethyl acetate and water
are 5 kmol/m3 and 30.7 kmol/m3,
respectively. The reaction is exothermic and it
is necessary to add a heat exchanger to keep its
temperature below 315 K. A coolant rate
sufficiently high that the ambient coolant
temperature is virtually constant at 290 K. Is
the exchanger adequate to keep the reactor
temperature below 315 K ? Plot temperature,
CA, CB, and CC as a function of time.
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V is a function of time
Mole balance
Rate laws
Energy balance
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Heat flow into the system (i.e. B and
W) therefore
This enery balance equation is solved together
with the mass balance equations to obtain the
temperature profile.
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In the previous discussion, the ambient
temperature was kept constant (i.e. Ta
constant). What is its effect when it does not
have a constant value?
Recall CSTR with a heat exchanger
The rate of heat transfer from the exchanger to
the reactor
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Energy balance on heat exchanger
Rate of energy in by flow
Rate of energy out by flow
Rate of heat transfer from exchanger to reactor
Eliminate Ta2
Back to the energy balance equation..
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Steady-state CSTR
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The startup of a chemical engineering reaction
process
It is often very important how temperature and
concentrations approach their steady-state values
in reactor startup.
Example
Propylene glycol is produced by the hydrolysis of
propylene oxide in a semibatch reactor with a
heat exchanger
Initially, there is only water at 75F and 0.1 wt
H2SO4 in a 500 gal reactor. The feed
stream consists of 80 lb mol/h of A, 1000 lb
mol/h of B containing 0.1 wt H2SO4, and 100
lb mol/h of M. Plot the temperature and
concentration of A as a function of time, and a
concentration vs. temperature graph for different
entering temperatures and initial concentrations
of A in the reactor.
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V is a function of time
Mole balance
Rate laws
Energy balance
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Heat flow into the system (i.e. A, B and
M) therefore
This enery balance equation is solved together
with the mass balance equations to obtain the
temperature profile. Page 555 558 show the
figures.