Data
Reactor unit operations in PRO/II use the reaction data capability. Click the Reaction Data button on the toolbar to define the reaction data prop-erties. You assign names to individual reactions and store groups of reac-tions in reaction sets for easy organization and retrieval. PRO/II comes with two predefined reaction sets, which contain data for the important Shift and Methanation reactions.
The Reaction Definitions dialog box in Figure 36 shows the contents of the user-defined reaction set, FURNACE_RXNS. The currently defined reactions are named H2BURN and METHBURN. Most reaction properties are entered through this dialog box. The , , and buttons allow you to input heats of reaction, equilibrium data, and kinetic data.
Figure 36:
Reaction Definitions Dialog Box
H... E... K...
PRO/II provides six types of reactor units that use reaction data supplied through the Reaction Data dialog box. Each requires a different subset of the reaction data and you only need to enter the required data for the specific reactor. Table 17 indicates which reaction data are required, R, and which are optional, O, for the six reactor types.
Reaction Stoichiometry
You can enter reaction stoichiometry by clicking on the Definition linked text. A table will appear that allows you to define your reaction by sim-ply filling in the blanks. PRO/II will even check your stoichiometry for errors. In Figure 36, the stoichiometry has already been defined for both reactions.
You should be aware that PRO/II selects the default reaction rate expres-sions based on the stoichiometric coefficients. For example, if you input a reaction as:
2A + B = 2C
PRO/II will generate the following rate expression:
rate1 = K [A]2 [B]
If, however, you enter the same reaction using the stoichiometrically-equivalent form:
A + 0.5 B = C
PRO/II will generate the following rate expression:
rate2 = K [A] [B]1/2
Clearly, a given reaction can have only one rate expression. The expres-sion is determined by the physics of the problem and should be indepen-dent of the way the reaction is written. PRO/II uses power law kinetics with the stoichiometric coefficients as the exponents. It is thus important that you enter your stoichiometry to reflect the actual reaction mecha-nism. If, for example, you know that this reaction exhibits tri-molecular
Table 17: Reactor Data
Reactor Type Data Type
Stoichiometry H E K
Gibbs O
Conversion R O
Equilibrium R O R*
CSTR or Boiling Pot R O R*
PFR R O R*
Batch R O R*
* These data can be entered through the reactor unit dialog box.
kinetics, then you should enter it as 2A + B = 2C and PRO/II will select the correct rate expression. If you choose not to do this, you are always free to manually override the expression PRO/II has selected for you.
Heat of Reaction Data
You can specify heat of reaction data by clicking that appears to the right of each reaction. In addition to the heat of reaction, you can also input the phase and temperature at which the measurement was taken. If you do not provide values, PRO/II will calculate the heat of reaction from heats of formation contained in its libraries.
Reaction Equilibrium Data
Clicking activates a dialog box that allows you to input reaction equilibrium data. The reaction equilibrium constant is:
(11)
where the ν's are the coefficients in the stoichiometric expression. By convention, the exponents are positive for products and negative for reactants. PRO/II expresses liquid activity in terms of mole fractions and vapor activity in terms of partial pressure. You can provide temperature dependent K-values by supplying coefficients (A,B,C,...) for the series:
ln(K) = A + B/T + Cln(T) +DT + ET2 + FT3 + GT4 + HT5 (12)
Reaction Kinetic Data
Clicking allows you to define the rate expression for the selected reaction. The rate expression has the form:
(13)
The temperature dependence is a corrected Arrhenius model, where:
■ A is the pre-exponential factor.
■ E is the activation energy.
■ n is the temperature correction exponent.
By selecting n=0, the standard Arrhenius form is recovered. Power law kinetics are used, and order is the reaction order for each component.
Click to enter the reaction order. As
you have just learned, if you do not provide component orders, PRO/II will use the stoichiometric coefficients from the reaction definition you provided.
Rate = Aexp[-E/RT]Tn activityiorderi
i
∏
Reaction Order & Activity Phase...
Kinetic Procedure Data
The Procedure Data function in PRO/II enables you to supply FOR-TRAN code for kinetic reaction rate calculations without the need for compilation and linking. Procedure blocks can be accessed in much the same way as FORTRAN programs call calculation subroutines.
The Procedure block has two main sections—declaration and code. The declaration section declares variable names, which are used in the code section but defined in the unit operation.
The code section contains FORTRAN-based statements which calculate the reaction rates. Specific unit operation data for the reactor or reactive distillation unit and reaction data can be accessed through number of predefined variables. Special intrinsic functions can be used in the code statements for direct retrieval of component properties
Accessing Reaction Data
After you enter your reaction data, you can select the appropriate reac-tion set from within each reactor unit dialog box. The two reacreac-tions,
H2BURN and METHBURN, will be listed in the Extent of Reaction dialog box, where you can define the order in which the reactions occur and to what extent.
Figure 37:
Conversion Reactor Dialog Box