Example: Batch reactor (BATCHREA) 

Six reactions of a batch reactor are modeled together with one algebraic balance equation of index 1.

The Mathematical Model:
The differential state variables are denoted by u1(t) , ..., u6(t), and the algebraic variable by u7(t) . Parameters to be estimated, are k1, km1, k2, k3, and km3. We define

u8(t)    =  E2 u1(t) /(E2 + u7(t) )
u9(t)    =  E3 u3(t)
/(E3 + u7(t) )
u10(t)  =  E1 u5(t)
/(E1 + u7(t) )

The six differential equations and the algebraic equation are

u1(t)t  =  -k2 u2(t) u8(t) 
(t)t  =  -k1 u2(t) u6(t)  + km1 u10(t) - k2 u2(t) u8(t)
u3(t)t = k2 u2(t) u8(t) + k3 u4(t) u6(t) - km3 u9(t)
u4(t)t = -k3 u4(t) u6(t) + km3 u2(t)
u5(t)t = k1 u2(t) u6(t) - km1 u10(t)
(t)t = -k1 u2(t) u6(t) - k3 u4(t) u6(t) + km1 u10(t) + km3 u9(t)
 -u7(t) + u6(t) + u8(t) + u9(t) + u10(t) = 0.0131

with initial values  u1(0) = 1.5776, u2(0) = 8.32  u3(0) = 0,  u4(0) = 0,  u5(0) = 0,  and  u6(0) = 0.0131. For u7(0), a consistent initial value can be found. The lower index t denotes the time derivative of the state variables.

1. Schittkowski (2002): Numerical Data Fitting in Dynamical Systems - A Practical Introduction with Applications and Software, Kluwer Academic Publishers
2. Caracotsios M., Stewart W.E. (1985): Sensitivity analysis of Initial values: Problems with mixed ODEs and algebraic equations, Computers and Chemical Engineering, Vol. 9, No. 4, 359-365

The complete solution of a data fitting problem is described in six steps:

  1. Define model type and document the experiment
    ... set some informative strings, define the mathematical structure and the variables
  2. Specify details of the model structure
    ... set number of differential and algebraic equations, tolerances, constraints, concentration values, ...
  3. Use editor for declaring variables and for defining functions
    ... the essential part, you have to know the mathematical equations and how to relate them to the format required by
  4. Insert measurement data
    ... the dirty job, can become boring (but you may import data from text files and EXCEL spreadsheets!) 
  5. Select termination tolerances and start a data fitting run
    ... only a few mouse clicks
  6. A separate process is started and all computed data are displayed
    ... MODFIT estimates parameters and performs a statistical analysis

Then you would like to take a look at reports and graphs:
    - parameter values
   - experimental data versus fitting criterion

Documentation and parameters:

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Model structure:

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Model equations (or use your own favorite editor):

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Measurement data (or use import function for text files or Excel):

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Parameters, tolerances, and start of a data fitting run:

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Numerical results (computed by the least squares code DFNLP):

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Report on parameter values, residuals, performance, etc. (or export to Word):

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Experimental data versus fitting criterion (also available for Gnuplot):

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