ChemCAD reactor simulation summary There are four types of reactor modules in ChemCAD:
Stoichiometric Used for single reactions where the user specifies the reaction stoichiometric stoichiometric coefficients, a key component, and a fractional conversion. The reactor module computes the outlet composition and heat duty or temperature rise.
Equilibrium Multiple reactions can be simulated in this module; the user specifies the extent of each reaction re action or the reaction equilibrium coefficient associated with each reaction. The simulation module then carries out the material and energy balances for the reactor. The user must specify the conversion or equilibrium coefficients in all cases except for the water gas shift (CO + H2O <==> CO2 + H2) and methanation (CO + 3H2 <==> CH4 + H2O) reactions; ChemCAD provides thermodynamic data for those reactions.
Gibbs Computes the equilibrium mixture composition given the reactor r eactor feed compsition and reactor operating conditions (e.g., T and P). All chemical species contained in the component list may participate in the reaction unless specified as inert. Equilibrium is determined by minimizing the total Gibbs energy of the mixture; thermodynamic data are provided in ChemCAD.
Kinetic All of the reactions in the reactors above reach their thermodynamic thermodynamic or specified conversion levels; normally, equilibrium will not be achieved in a real reactor system of finite size or residence re sidence time. Reactor size in design calculations must be determined using reaction rate data and a kinetic reactor module is used to determine a reaction conversion rate for fixed reactor size, or the necessary reactor size given a specified conversion. Reaction rate constants, activation energies, and other reaction rate data must be supplied by the user.
ChemCAD reactor design tutorial Consider a hydrogen/nitrogen feed stream with the following properties
Temperature 500 K Pressure 100 atm Composition 20 mole% N2, 80 mole% H2 Flow rate 1000 kgmol/hr In this sequence of ChemCAD design examples, we examine several different approaches to designing a reactor to carry out ammonia production by the Haber process: N2 + 3H2 <==> 2 NH3 The reactor specifications are as follows Adiabatic reactor Pressure 100 atm Pressure drop 1 atm Molar conversion of N 2 0.5
Stoichiometric/Equilibrium reactor design procedure 1. Start the ChemCAD application; choose menu item File -> New Job and save as a folder on your computer's desktop. 2. Select the arrow from the top-left corner of the main palette, then click on Stoichiometric or Equilibrium reactor. Click add main symbol and place the reactor. 3. Then place, by selecting from the main palette, the input arrow (red arrow) and output arrow (purple arrow). Place the arrows accordingly near the reactor. 4. Next use the stream button (second row, and second column of the main palette) to connect the input arrow to the top of the reactor, and the bottom of the reactor to the output arrow; blue dots represent equipment feed streams and red dots represent equipment outlet streams. 5. Change the pull down menu on the toolbar from flowsheet to simulation. 6. Select Thermophysical -> Component List and add nitrogen (46), hydrogen (1) and ammonia (63). Click OK until you are through all the menus that come up. 7. Choose menu item Format -> Engineering Units to select SI units for this problem; component flows should be mole fractions.
Also, change pressure to atmospheres. After making these choices, click OK. 8. Double-click on the square input stream box to specify input stream properties. Enter the feed properties listed above. 9. Next, double-click on the reactor. For the Stoichiometric reactor specify the following: Specify thermal mode to adiabatic; Select nitrogen as the key component; set fractional conversion to 0.5; Specify the following stoichiometric coefficients: hydrogen: 2; nitrogen: -1; ammonia: 2; Set reactor pressure to 100 atm. 10. The stoichiometric reactor can only specify one reaction; for more reactions, one must use the equilibrium reactor module. For the Equilibrium reactor specify the following: Specify the number of reactions as 1; Set pressure and pressure drop; Specify the reaction to take place in the vapor phase; Specify thermal mode to adiabatic; Choose reaction conversion, then [OK] Select nitrogen as the key component; set fractional conversion to 0.5; Specify the following stoichiometric coefficients: hydrogen: 2; nitrogen: -1; ammonia: 2; 11. From the main menu, select Run -> Run All; correct errors, if any. 12. From the solution report obtained from Output -> Report click on the Calculate and Give Results button. o o
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Note that reaction information in the Equilibrium reactor is similar in form to that used for reactive distillation in the SCDS (distillation) column module.
Gibbs reactor design procedure 1. Start the ChemCAD application; choose menu item File -> New Job and save as a folder on your computer's desktop. 2. Select the arrow from the top-left corner of the main palette, then click on Gibbs reactor. Click add main symbol and place the reactor. 3. Then place, by selecting from the main palette, the input arrow (red arrow) and output arrow (purple arrow). Place the arrows accordingly near the reactor. 4. Next use the stream button (second row, and second column of the main palette) to connect the input arrow to the top of the reactor, and the bottom of the reactor to the output arrow; blue
dots represent equipment feed streams and red dots represent equipment outlet streams. 5. Change the pull down menu on the toolbar from flowsheet to simulation. 6. Select Thermophysical -> Component List and add nitrogen (46), hydrogen (1) and ammonia (63). Click OK until you are through all the menus that come up. 7. Choose menu item Format -> Engineering Units to select SI units for this problem; component flows should be mole fractions. Also, change pressure to atmospheres. After making these choices, click OK. 8. Double-click on the square input stream box to specify input stream properties. Enter the feed properties listed above. 9. Next, double-click on the Gibbs reactor. Specify the thermal mode to adiabatic. Set the reaction phase to [1 Vapor or mixed phase] and press OK and OK at the Component Element Matrix. 10. From the main menu, select Run -> Run All; correct errors, if any. 11. From the solution report obtained from Output -> Report click on the Calculate and Give Results button.
Kinetic reactor design procedure - PFR with zeroeth order reaction 1. Start the ChemCAD application; choose menu item File -> New Job and save as a folder on your computer's desktop. 2. Select the arrow from the top-left corner of the main palette, then click on Kinetic reactor. Click add main symbol and place the reactor. 3. Then place, by selecting from the main palette, the input arrow (red arrow) and output arrow (purple arrow). Place the arrows accordingly near the reactor. 4. Next use the stream button (second row, and second column of the main palette) to connect the input arrow to the top of the reactor, and the bottom of the reactor to the output arrow; blue dots represent equipment feed streams and red dots represent equipment outlet streams. 5. Change the pull down menu on the toolbar from flowsheet to simulation. 6. Select Thermophysical -> Component List and add nitrogen (46), hydrogen (1) and ammonia (63). Click OK until you are through all the menus that come up. 7. Choose menu item Format -> Engineering Units to select SI units for this problem; component flows should be mole fractions. Also, change pressure to atmospheres. After making these choices, click OK.
8. Double-click on the square input stream box to specify input stream properties. Enter the feed properties listed above. 9. Next, double-click on the Kinetic reactor and specify the following: On page 1 of the specifications set the number of reactions to 1, pressure to 100 atm, pressure drop to 1 atm, and select Standard kinetic expression; Specify the reactor as a PFR with vapor phase reaction only, adiabatic operation; Set the switch to Specify volume, Calculate conversion and set the reactor volume to 10 m 3; On page 2 set the Frequency factor to 5 and Activation energy to 1e-20 (CHEMCAD will treat these essentially as zero); leave all other reaction parameters blank except the stoichiometry coefficients which should be set to -3 for hydrogen, -1 for nitrogen, and 2 for ammonia; for these components, set the respective Exponential coefficient to 1e-20 (again, to force the values to zero); On page 3 (More specifications) set the reaction units to: Concentration flag to moles/vol, volume to m3, time to hrs, Activation energy to K Joules, molar flow to K moles, and mass flow to kg. 10. From the main menu, select Run -> Run All; correct errors, if any. 11. From the solution report obtained from Output -> Report click on the Calculate and Give Results button o
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