The equations that define the chemical process are shown below. The reactants, A and B, are mixed together while carbon dioxide is continuously added to produce a desired third compound C. In the interest of commercial competition, the names of the chemical species have been changed.

We can define the above reaction scheme as a mathematical system, using the equation below:

,

with  and   

The matrix, M, is of rank 5 and given by:

  and the function f by:

where the and  are auxiliary variables given by:

The variables , , , , , and , are the concentrations of the following compounds , , , ,  and , respectively. The partial pressure of  is given by  and H is Henry's law constant.  

Using the information above, we can derive the system of nonlinear differential-algebraic equations (DAEs) that models the chemical process described above.

Rearranging the equations derived above, we end up with a stiff system of 6 non-linear DAEs of index 1. Although there is no single formulation, stiffness is usually defined as an equation (or system of equations) with terms that can lead to rapid variations in the solution.  

 =

 =

 =  

 =  

 =

 =  

We now create a system model containing the equations derived above and the initial conditions.

The values of the physical parameters and initial conditions of the system are given by:

We can now solve the DAE system and plot how the concentrations of the various compounds change with time using the  command

The discrepancy between the actual and expected concentration of compound C was revealed by comparing the experimental reaction kinetics for each compound to the predicted values.

The experimental data, stored in an Excel file, was extracted using the following commands:

The following commands extract the values from Excel:

The following plots show the experimental versus predicted reaction kinetics for each compound. From these results, it is apparent that the model derived in the previous section does not adequately predict the experimental results for compound A and compound C.

In fact, the concentration of compound C at steady state is , which is 1.5 times lower than the steady state concentration observed via the experimental data. Similarly, the concentration of compound A at steady state is , which is 1.3 times smaller than the steady state concentration obtained via the experimental data.    

 =

 =  

The statistical calculations (mean, standard deviation, and root-mean square error), confirm the presence of an error in one of the rate constants controlling the concentration of compound A. The data, summarized below, show the root mean square error of compound A to be 0.00301799 which is several orders larger than the root mean square error values of the other compounds.

A better prediction for the reaction kinetics of compound A can be obtained by parameterizing the numerical solution for the differential equation obtained earlier, and fitting the theoretical model to the experimental data using the least-squares approach.

The following procedure calculates the sum of the squares of the error between the experimental data and the model prediction for the rate constant . It assumes that the only uncertainty in the experimental data exists in the concentration, . In reality, the time the reading was taken will also be subject to a degree of error. If this was to be taken into account, then the following procedure can be easily modified to calculate the sum of the short orthogonal distance between the data points and model predictions; this is also known as orthogonal regression.

The value of  that best describes the experimental data is

 =

This value for  is much smaller than the value of  that was used to develop the original model (that is, 10.925 vs. 28.7). 

Taking into account the new value of  as shown below, the theoretical model accurately predicts the experimental results.

 =

With this new rate constant, the concentration of compound C now more accurately matches the concentration being produced at the plant.