ChemCADMichael 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

Workspace

Toolbox

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

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

H2S-rich gas

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 component-element 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.