Two methods can be used for the gas phase reactions. In the first one reactions are not important, but reagents, and we look for the equilibrium composition by minimizing Gibbs free energy. In the second one you need to specify the system of linearly independent chemical reactions and know the values of equilibrium constants. Further, Guldberg-Waage law, and then the solution of a non-linear system of equations with respect to conversion degrees. For a liquid phase reactions or a more complex system, it is better to perform the experiment.

THE PROCEDURE FOR FINDING EQUILIBRIUM CONVERSION OR EQUILIBRIUM COMPOSITION IF SPECIES IS AVAILABLE ON: OCTAVE, LEVENSPIEL .(2001). CHEMICAL REACTION ENGINEERING. 3RD EDITION,SINGAPORE: JOHN WILEY & SONS, PP 211-215, CHAPTER 9: TEMPERATURE AND PRESSURE EFFECTS.

The mathematical model RGibbs in Aspen Plus Software simulates equilibrium of a system by minimizing Gibbs energy. You can use this model to enter your components and expected reactions (no kinetic data required) and run a sensitivity analysis for reactant(s) conversion(s) by menas of various parameters, i.e. temperature.

I hope that helped.

Fache Axel

Thanks Mirosław Grzesik.

That's helpful Idongesit Effiong Sampson. Currently studying it.

Georgios Varvoutis could you elaborate this process in step-wise manner to someone using Aspen Plus for the first time?

You need to define your components, your component properties method (SRK is usually satsifactorily accurate), all possible reactions, your initial conditions (temperature, pressure) and then simulate the stream using the RGibbs reactor, which is a mathematical model that simulates an infinitely long reactor, i.e. thermodynamic equilibrium conditions with minimum Gibbs energy of the system. Using Sensitivity analysis, you can further plot many variables using different parameters. Aspen Plus is a tough program to start working with, though.

Thanks Georgios Varvoutis. I figured it out by hand calculations

As mentioned by other colleagues, two approaches are available:

1. Gibbs energy minimization. In this approach all you need is the starting reaction mixture and their conditions i.e. T and P and composition. This typically yields a set of simultaneous nonlinear algebraic equations.

2. Equilibrium constant method where you formulate the "independent" reactions and their equilibrium constants from the law of mass action. This is also a simultaneous set of nonlinear equations,

In both methods, if you want to have the influence of conditions engaged then you need to include the energy balance as an additional equation.

At relatively law pressures you may use ideal gas law but at high pressures then instead of partial pressures you need to work with fugacity of species involved.

Much chemical process modelling and/or simulation work can be done with the widely-used Microsoft’s Excel.

Application example ― Simulations carried in Excel 5.0 with Visual Basic for Applications (VBA) macros ― The recursive least squares algorithm (RLS) allows for (real-time) dynamical application of least squares regression to time series. Years ago, while investigating adaptive control and energetic optimization of aerobic fermenters, I have applied the RLS algorithm with forgetting factor (RLS-FF) to estimate the parameters from the KLa correlation, used to predict the O2 gas-liquid mass-transfer, while giving increased weight to most recent data. Estimates were improved by imposing sinusoidal disturbance to air flow and agitation speed (manipulated variables). The power dissipated by agitation was accessed by a torque meter (pilot plant). The proposed (adaptive) control algorithm compared favourably with PID. Simulations assessed the effect of numerically generated white Gaussian noise (2-sigma truncated) and of first order delay. This investigation was reported at (MSc Thesis):

Thesis Controlo do Oxigénio Dissolvido em Fermentadores para Minimi...