Welcome to the Aspen Plus V8.6 teaching module on Free Energy minimization with RGIBBS.
For information on navigating this module, please refer to Navigation Hints located above the slide. Click the Next button on the bottom right hand corner to begin.

Gibbs Free energy is the maximum amount of non-expansion work that can be extracted from a closed system. It is the chemical potential that is minimized when a system reaches equilibrium while maintaining a constant temperature and pressure. Gibbs Free Energy, simply known as Gibbs Energy, depending on your source, was originally called “Available energy”, since it is the energy from a reaction that can be readily put to use.

The change in Gibbs Free energy for an isolated chemical system at a constant temperature and pressure is defined as the change in enthalpy minus Temperature multiplied by the change in entropy. If the Gibbs Free energy change is less than 0, the reaction is favorable or spontaneous. If it is greater than 0, the reaction is disfavorable, or not spontaneous.

Exothermic reactions in which entropy decreases are often labeled as “Enthalpy Driven”. Endothermic reactions in which the 𝑇∆𝑆 term is greater than ∆𝐻 are often labeled as “Entropy Driven”.

The formation of ammonia is a classic example of an enthalpy driven reaction. The reaction is exothermic but there is a decrease in entropy since the reaction reduces the total molar count of by ½. At room temperature, the contribution of the entropy term is only about 2/3 the magnitude of the enthalpy change so the Gibbs Free energy is still negative.

It is important to note that reaction spontaneity does not indicate reaction speed. At room temperature, the formation of ammonia does not occur at a noticeable rate. The reaction rate increases with higher temperatures but the favorability of the reaction quickly diminishes.

The reaction of Barium Hydroxide octa-hydrate and ammonium chloride is a commonly used example of an endothermic, entropy driven reaction. With the formation of aqueous ammonia and liquid water comes a large increase in entropy. Even at very low temperatures, the entropy change is greater than the change in enthalpy, so it drives the reaction. In chemistry labs, the reaction beaker is placed on a wet surface to demonstrate heat transfer when the water freezes.

We can use the RGibbs reactor to model chemical and phase equilibrium in Aspen Plus. The RGibbs reactor minimizes Gibbs Free Energy, subject to atom balance constraints. It does not require stoichiometry and will consider equilibrium between all components. If the user prefers, they can restrict equilibrium by specifying fixed molar amounts of a given product, percentage of feed that does not react, or the approach temperature to equilibrium of the system or any number of reactions. Any number of phases are allowed with the RGibbs reactor and the user is given the ability to assign components to particular phases. It is important to emphasize that this reactor does not consider rate; All components are simply brought to equilibrium for the specified conditions, subject to constraints.

For this module, we will be modelling combustion equilibrium as you might expect to see in a gasoline engine. We will be using isooctane and an approximate air mixture at stoichiometric equivalent. In actuality, gasoline is a complex mixture of over 500 hydrocarbons, commonly between 5 and 12 carbons in size, so isooctane is just an approximation to illustrate the equilibrium behavior. Since modern gasoline engines have a typical compression ratio of around 10:1 and often are turbocharged increasing the pressure further, we will be using high pressures to demonstrate conditions under load.

We will start by specifying all common components of isooctane combustion and specify the property method. Next we will specify an RGibbs reactor for no heat loss and high pressure. We will then specify our fuel streams and run the simulation to determine the chemical equilibrium of combustion gasses at combustion temperature.

Start a new blank Aspen Plus Simulation.

Enter the following common components of combustion. You will need to add the APV86 Combust databank to the list of selected databanks for all components to register.

Go to the Methods | Specifications sheet and select the Ideal method.

Now we will enter the simulation environment to create our flowsheet.

Add an RGibbs reactor to the flowsheet. Double Click on the RGibbs Reactor and specify pressure as 18 atm and heat duty as 0.

Connect 2 material streams, one labeled “AIR” and one labeled “FUEL”. Specify the fuel stream as 1 kmol/hr isooctane at 298.15K and 1 atm pressure. Specify the air stream as 12.5 kmol/hr O2, 47.5 Kmol/hr N2, 298.15K and 1 atm pressure. The two streams together represent the approximate mixture for isooctane combustion with no excess air.

Connect a products stream from the RGibbs reactor. We are now ready to run the flowsheet.

Double clicking on the products stream brings us to the results page, as we can see, there is a measurable concentration of all components except isooctane. It should be noted that at this temperature there are high concentrations of harmful chemical such as Carbon Monoxide and Nitrous oxides. This is the reason why vehicles use catalytic converters to reduce harmful emissions. By allowing exhaust to reach equilibrium at lower temperatures with a platinum catalyst, before exhaust exits the vehicle, we greatly reduce emissions.