ChemCAD
Michael Naas, Teddy Wescott, Andrew Gluck
What can ChemCAD do?

Able to model many typical chemical engineering processes in order to
produce results based on user specified input

Help make decisions regarding sizing and costing equipment

Allow the user to select the thermodynamic model best suited to the process
conditions

Save chemical engineers time by performing many complex calculations based
on a few parameters

ChemCAD WILL NOT be able to give you the right answer if you make bad
assumptions or if you use the wrong process design
ChemCAD User Interface
Toolbox 
Workspace
Directory
ChemCAD Generated Reports

ChemCAD gives the answers to
your simulations in reports and
graphs

The reports will give all
thermodynamic values based on
the model you chose along with
flow rates and compositions

The graphs can show things such
as the temperature downstream
in a heat exchanger and such
A Few of The Major Tools:

Reactors

Heat Exchangers

Separation Equipment

Distillation Columns (or flash)

Absorbers

Mixers

Condensers

Piping and Feed/Product Streams
The Claus Process in ChemCAD
H2S-rich gas


The reactions in our project
come from the Claus Process

H2S + 1.5 O2 ---> SO2 + H2O

2 H2S + SO2 ---> 3S + 2 H2O

CH4 + 2 O2 ---> CO2 + 2 H2O
These reactions (and possibly
other undesired side reactions)
will occur simultaneously in
each reactor
Elemental sulfur
Reactor Modelling in
Chemcad
Introduction to Reactor Modelling in
Chemcad


Several different types of reactor unit ops are available in chemcad.

Stoichiometric

Equilibrium

Gibbs

Kinetic
Each class of reactor takes different specifications from the user and models
the reaction based upon the specified parameters.

Different reactor models can give different results for the same conditions.
The Reactors


Stoichiometric Reactor

Only can be used for a single reaction.

User specifies the stoichiometric coefficients of the reactants, a key component
and the fractional conversion of the key component.

Reactor computes the outlet stream composition and temperature as well as the
heat duty.
Equilibrium Reactor

Can be used to model multiple reactions.

User specifies the reaction equilibrium coefficients for each reaction (Keq’s).

Chemcad computes outlet composition by completeing mass and energy balances
based off of input streams + reactions along with Keq’s.
The Reactors cont.


Gibbs Reactor

User specifies operating conditions of the reactor (T,P, components, flow rates).

Chemcad computes the output stream composition as well as its temperature and
pressure by allowing all chemical components to participate in potential reactions
and minimizing the Gibbs free energy of the output stream.
Kinetic Reactor

User supplies rate constants, activation energies, specifies each reaction that is
expected to occur (up to 20 simultaneous reactions may be specified).

Chemcad calculates conversion for a specified reactor size or reactor size for a
given conversion.
Example: Gibbs vs Kinetic. Reactor for
conversion of toluene to benzene.

For an input stream composed of methane, hydrogen, toluene, benzene, and
biphenyl we consider the following two reactions concerning the conversion
of toluene.

The input stream composition is given by
Chemcad analysis:
Kinetic Reactor set-up

Chemcad requires user input for
number of reactions, reactor
pressure, operating mode (adiabatic,
isothermal, etc…), reactor type (PFR
or CSTR), and the phase (liquid,
vapor) that the reaction is expected.

Can choose between specifying key
component and conversion, and
calculating reactor volume or
specifying volume and chemcad then
calculates conversion.
Kinetic Reactor set-up cont.

Requires the
stoichiometric
coefficients for reactions
considered.

Also requires activation
energy (Ea) and frequency
factor (ko) for each
specified reactions.

Only will consider
reactions specified by
you.
Reaction Kinetics review

Ko is the frequency factor, has its
origin in statistical mechanics and
collision theory may be found by
fitting data to exponential equation
or modeled theoretically by
molecular modeling.

Ea is activation energy, can often be
approximated through use of Gibbs
free energy of formation for each
chemical component.

Temperature dependency can be
approximated by Van hoff equation
Gibbs Reactor set-up

User must specify thermal
mode, reaction phase, reactor
operating pressure, expected
pressure drop across the
reactor.

Will calculate heat of reaction,
can also input inert spectator
species which impact
pressure/volume and take up
heat but do not react.
Gibbs Reactor set-up

Chemcad will compute componentelement matrix but asks you to
verify before proceeding.

Chemcad does this so that it may
consider all possible reactions
between the specified input
species and then computes outlet
stream composition/conditions by
attempting to minimize the Gibbs
free energy of the products.
Results: Gibbs vs. Kinetic

If you look carefully you can see that although the inputs are identical the outputs are
different.
Results: Gibbs vs. Kinetic

The kinetic reactor’s mass balance and energy balances are way off so there is probably
something wrong with the kinetic model used for the reactor.

The Gibbs reactor which relies more heavily on thermodynamics works out well balance wise,
thermodynamic model is probably accurate. In reality however we do not know how close
the reaction comes to thermodynamic equilibrium.