Learning R using a Chemical Reaction Engineering Book: Part 2

January 26, 2013

(This article was first published on Notes of a Dabbler » R, and kindly contributed to R-bloggers)

In case you missed part 1, you can view it here. In this part, I tried to recreate the examples in section A.2.2 of the computational appendix in the reaction engineering book by Rawlings and Ekerdt.

Solving a nonlinear system of equations

This example involves determining reaction equilibrium conditions by solving the following system of nonlinear equations.
\begin{aligned}  PK_1y_Iy_B-y_{P1}&=&0, \\  PK_2y_Iy_B-y_{P2}&=&0  \end{aligned}

The relation between the variables y_I,y_B,y_{P1},y_{P2} and extent of reactions x_1,x_2 are:
\begin{aligned}  y_I&=&\frac{y_{I0}-x_1-x_2}{1-x_1-x_2} \\  y_B&=&\frac{y_{B0}-x_1-x_2}{1-x_1-x_2} \\  y_{P1}&=&\frac{y_{p10}+x_1}{1-x_1-x_2} \\  y_{P2}&=&\frac{y_{p20}+x_2}{1-x_1-x_2}  \end{aligned}

Here I have used R package rootSolve for solving the above set of equations to determine x_1 and x_2 . The library is loaded and the functions to be solved are defined in the R function fns.

# load library rootSolve

# function defining F(x)=0
K1=108; K2=284; P=2.5
yI0=0.5; yB0=0.5; yP10=0; yP20=0;

Next, an initial guess of (0.2,0.2) is set for the variables and the equations are solved using the function multiroot (from package rootSolve)

# initial guess for x

# solve the equations

# object returned by multiroot
> xans
[1] 0.1333569 0.3506793

F1           F2
6.161738e-15 1.620926e-14

[1] 7

[1] 1.11855e-14

# solution to the equations
> xans$root
[1] 0.1333569 0.3506793

The solution to the equations is accessed from the variable xans$root which in this case is (0.1334,0.3507)

MATLAB/Octave functions for solving nonlinear equations (fsolve) have been used in Chemical Engineering computations for a long time and are robust. R has traditionally not been used in this domain. So it is hard to say how the functions I have used in this blog will perform across the range of problems encountered in Reaction Engineering.

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