Reactor Modelling in Chemc
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.
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