Case studies - chemical equilibria

Introduction

Most thermodynamic simulation in process engineering centres on the need to calculate phase equilibria for separation processes, especially distillation. However chemical equilibria is another area where thermodynamic information can be exploited to make predictions about chemical processes.

The chemical reaction module in Multiflash is a utility for performing simultaneous phase and chemical equilibrium calculations. It can handle equilibria involving combinations of one gas phase, one liquid phase and any number of pure solids.

The chemical reaction module does not rely on reaction schemes. You do not need to specify any reaction mechanism but only list all the possible products and reactants.

The applications are many and varied but a sample is discussed here

Xylene isomerisation

To start with a very simple example, select ortho-, meta- and para-xylene from the INFODATA fluids bank.

Select RKS using the route Select/Model set/Equations of State/RKS. The model set will be used to predict the phase properties and any of the cubic equation of state models would be acceptable for this case study.

Enter a temperature of 575K and a pressure of 10 bar (remember to set the pressure and temperature units correctly).

Click on the Chemical reaction button,

The mixture will be in the gas phase. You may ignore the warning message. All model sets and model configuration files include phase descriptors for several phases to give the user greatest flexibility. However, the chemical reaction module is limited to one gas and one liquid phase, hence the warning message and the ?CONVERGED message. If you use the problem setup file, xylene.mfl, only one liquid phase is defined and the warning message will not appear. Also, the calculation will be reported as converged.

Input any composition for the mixture provided there is a positive amount for each xylene.

At the same conditions the ratios of the three xylenes will remain the same.

If you reduce the temperature to 300K and the pressure to 1 bar the mixture will be in the liquid phase. The equilibrium concentrations of the three xylenes are slightly, but not significantly, different and again are unaffected by the input compositions

Steam cracking of ethane

This case study to investigate the steam cracking of ethane is effectively a study of two competing reactions

2H2O + C2H6 = 2CO + 2H2O

4H2O + C2H6 = 2CO2 + 7H2

You do not have to specify the reaction schemes only the products and reactants. Therefore, you should

Select ethane, water, hydrogen, carbon monoxide and carbon dioxide from INFODATA

In the drop down table under Composition enter 2 moles of water and 1 mole of ethane

Select an equation of state model, e.g. RKS

Set the temperature to 1000K and the pressure to 1e5 Pa.

For this calculation it may also be useful to change the output from fractions to amounts, see “Changing units” on page 152.

Click on the chemical reaction button,

Under these conditions 4.96 moles of hydrogen, 1.974 moles of carbon monoxide and 0.00668 moles of carbon dioxide are produced.

Vary the ratio of water to ethane and note the increase in the amounts of carbon dioxide as the ratio increases.

You can look at the enthalpy changes for the reactions by carrying out a simple P,T flash to obtain the enthalpy of the reactants and the chemical P,T reaction flash to see the enthalpy of the products.