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Reactor Design and Safety Requirements
Information about reactor design
Fluidized-bed reactors
Bioreactors
Safety considerations
Simplified cost
estimating
When a reactor is either heated
or cooled, cost it as the sum of the cost of a heat exchanger plus that of a
pressure vessel. For a plug flow
reactor, calculate the area from the tube diameter, length and numbers. For a fluidized bed reactor use the Q and Ts
to calculate the area as for other heat exchangers. For a stirred-tank reactor, add in the cost
of stirring equipment.
Modeling using HYSYS or UniSim
Warning about Aspen HYSYS 2006
Many
of our students have encountered problems while modeling heterogeneous
plug-flow reactors in HYSYS 2006 using the NRTL properties. These problems may be peculiar to the
academic version we have installed.
Since Aspen no long provides assistance to academic licensees, you’re on
your own if you have similar problems.
I’d be interested in hearing the experience of other people. Following are the problems our students have
had, and how we have worked around them:
Types
of reactors in HYSYS & UniSim: Refer to the Reactor sections of the
AspenHYSYSOperationsGuide.pdf documentation included with HYSYS and probably to
be found at C:\Program Files\AspenTech\Aspen HYSYS
2004.2\Documentation\. HYSYS has six types of idealized reactor models built in:
conversion, equilibrium, Gibbs, yield shift, plug flow, and continuous stirred
tank (CSTR). The first four do
not require reaction kinetics, but provide no information about the size of the
reactor and, therefore, its cost. The
plug flow reactor and CSTR modules both produce size information, but require
that at least the forward reaction kinetics be known.
When
entering the reaction kinetics in the basis environment, note that, in general,
the reaction orders of the components are NOT the same as their stoichiometric
coefficients. Use the powers on the
basis (concentration, mole fraction, etc.) for the actual reaction kinetics
specified or taken from the literature.
For example if the rate is proportional to CA0.5
then the basis is concentration and the order for component A is 0.5. If you are given only forward reaction kinetics
and suspect that the reaction may be reversible, you can check using the method
at calculation
of equilibrium constants and reverse reaction kinetics. (Derivation of
expression for reverse reaction rate.)
The reactions and the percent
conversion of a component must be given.
The conversion can be specified as a function of temperature.
The reactions must be specified
and their equilibrium constants either given or calculated by HYSYS using the Gibbs free energy
of the reaction at standard conditions.
The products are given, without specifying the
reactions that would give those products.
HYSYS then calculates the equilibrium composition of the product stream
by minimizing its Gibbs free energy, constrained by conservation of atomic species
using the composition of the feed stream.
From the HYSYS documentation, “As
with the Equilibrium Reactor, neither pure components nor the reaction mixture
are assumed to behave ideally.”
From the HYSYS
documentation, “The Yield Shift reactor unit operation supports efficient
modeling of reactors by using data tables to perform shift calculations. The operation can be used for complex
reactors where no model is available, or where models that are too
computationally expensive.” This is
beyond the scope of an introductory chemical engineering design course.
A packed bed catalyst will be
inside the tubes, with a heat-transfer medium flowing co-current outside the
tubes. You should set only the inlet T
and P of the process stream; HYSYS will calculate the outlet T and P. (The outlet pressure must be less than
the inlet pressure.) Recommended Design
Parameters: Ergun, Cooling by formula.
Design Heat Transfer: SS duty by formula, Heat medium (for an
endothermic reaction; coolant for exothermic) given the wall heat transfer
coefficient, Mole flow of heat-transfer medium, Heat capacity of the medium,
and its Inlet T. The heat-transfer flow
and inlet T should be adjusted to give reasonable operation. Usually the Tube side (catalyst) heat
transfer will be specified. If there is a hot spot near the beginning of an exothermic reactor,
set the Reactions Overall to Re-init.
If the pressure drop is too large, increase the reactor volume without
increasing the tube length (i.e., add more tubes to decrease the stream
velocity). Record the Q and the coolant
temperatures in and out for use later.
See the reference to Ulrich and Vasudevan
above for typical values of operating parameters.
·
Procedure for
simulating cooling of a plug-flow reactor with a boiling liquid (Similar
for heating with condensing steam)
Last revised July 28, 2009. Please submit all questions, comments and
suggestions to W.R.
Wilcox
Disclaimer: The material on these pages is intended for instructional
purposes by Clarkson University students only.
Neither Clarkson University nor Professor Wilcox is responsible for
problems caused by using this information.
Wilcox
home × ChE
design home × Profession × General × Properties × Equipment × Separation × Aspen + × HYSYS &
UniSim × Costs × Safety × Case studies × Excel
× MATLAB × Experiment Design