This article was originally published on March 26, 2020.
(Image courtesy of Chemstations.)
When it comes to scaling up chemical production processes, simulations are a necessary and economic way to size equipment and piping. Chemical engineering simulations are required for the chemical processing industry to test out different reaction systems before investing large amounts of money in pilot or demonstration plants.
As a chemical engineering college professor, I can speak to the importance of simulation for students to see the actual sizes of the piping and reactors needed to make a process economical. As an instructor currently in the middle of a semester of chemical engineering laboratory classes, I can testify that simulations are an especially necessary teaching tool during the COVID-19 pandemic. Without physical access to my teaching lab with heat exchangers and pipe flow systems, I am intending to greatly incorporate simulation tools into the instruction of these up-and-coming engineers.
Chemstations CHEMCAD
(Image courtesy of Chemstations.)
There are several simulation programs out there, but the program that I use is CHEMCAD, developed by Chemstations. CHEMCAD is a chemical process modeling software designed to help chemical engineers simulate real world systems. As someone who educates the next generation of engineers, I have always found CHEMCAD to be an excellent program. However, as with any simulation program, there is a learning curve to mastering it.
Here, I’m going to provide tips for how to select reactors for reaction system simulations, and how to overcome some common hiccups when working with different reactors.
First, before you can utilize any reactors, you need to input the different chemicals involved in the engineering system that you are trying to simulate. To do this, you need to ‘Select Components’ to be used in your system. There are over 2400 different chemicals available in CHEMCAD, and users can add their own chemicals as well.
In addition, it is important to choose the thermodynamic models that you want CHEMCAD to use when analyzing your simulation. This is a key step, as different models can result in very different simulation results. A thorough understanding of the different K-value models and enthalpy models is needed to make an educated choice in your simulation. K-value models use equations of state to determine the liquid-vapor phase equilibrium of the system, and the enthalpy models are used to determine heat balances in the system. Luckily, once you have input the chemicals in your system, CHEMCAD will recommend the K-value and enthalpy models that are likely best suited for your system. However, as an engineer, you do need to use your own judgement to confirm if these are valid models for your system.
Reactor Types in CHEMCAD
Figure 1. An Equilibrium, Gibbs, Kinetic, and Stoichiometric Reactor all in a row. There are different reactor styles to choose from as shown in the pop-up box.
CHEMCAD offers many different types of reactors to use, but their names do not always make it clear as to which reactor you should choose for your situation. Available choices are an equilibrium, Gibbs, kinetic, and stoichiometric reactor. In addition, each of these reactors looks identical, unless you choose a different reactor style option (right mouse click over a Unit Operation). While most of these reactors can be used interchangeably, each of these reactors is best suited for different situations and each reactor requires different inputs.
For example, the stoichiometric reactor is best to be used with simple reactions. In this reactor, you need to specify the stoichiometric coefficients for each of the chemicals involved in the reactor’s reaction to run a simulation. The thermal mode of the reaction, whether it is adiabatic (constant heat), isothermal (constant temperature), or it has a specific heat duty, also must be input.
Figure 2. A Stoichiometric Reactor in CHEMCAD.
Sometimes multiple reactions are taking place in a reactor, and this is where an equilibrium reactor will shine. Here, you first need to specify the number of reactions taking place, as well as define the physical state of those reactions (liquid, vapor, or a combination) and the thermal mode for those reactions. After these details are provided, the user will be asked to input more information about each reaction, including stoichiometric coefficients. In reality, an equilibrium reactor is just a stoichiometric reactor that can handle a multi-reactive system.
Figure 3. An Equilibrium Reactor in CHEMCAD.
The Gibbs reactor is best used for trying to estimate a hypothetical equilibrium mixture, so if you are unsure about what reaction might be taking place, this is your best place to start. It is also in this reactor that you can specify what input chemicals will be solid at the outlet of the reactor, and you can specify the inert (unreactive) species in the system.
Figure 4. A Gibbs Reactor in CHEMCAD.
The last reactor option is a kinetic reactor, which is meant to be used when you want to model a continuous stirred tank reactor (CSTR) or plug flow reactor (PFR). A CSTR is a batch reactor that contains a stirrer to allow for a through mixing of reagents, while a PFR is a usually long, cylindrical reactor used with continuous flow systems. As these are very specific systems, this reactor has particular uses and is not as interchangeable in your system as the other three reactors.
Figure 5. A Kinetic Reactor in CHEMCAD.
How to Simulate Catalysts
One frequent issue in CHEMCAD is how to simulate a catalyst. Unfortunately, to my knowledge, there is no catalyst setting within this program. However, there are workarounds for catalysts when using these different reactors.
Rather than adding a catalyst as a chemical species in the system, the user can use either the equilibrium or stoichiometric reactor to specify the exact stoichiometric coefficients for the reactants and products, and by doing this, the user can digitally control the reaction simulated in the reactor. The other option for working in a catalyst in a Gibbs reactor is to specify inert species. If the presence of a catalyst brings about one chemical reaction over another potential reaction, the user can specify that the reactant involved in the reaction that is not encouraged by the catalyst is an inert species. Even if this is not true by the definition of the word ‘inert,’ as this reactant is not favored, we can trick CHEMCAD into ignoring this reactant in favor of the catalyst-preferred reactant.
CHEMCAD’s troubleshooting advice is very useful; CHEMCAD points out the unit operation within your simulation where an error is occurring. In Figure 6, I did not enter any specifications for one of the reactors, and when I tried to run the program, an error message popped up highlighting this exact unit operation so that I could quickly find and correct the error. Overall, the easiest way to create a working simulation is to add one piece of equipment to your system at a time, so that you can troubleshoot individual pieces of equipment in sequence, rather than troubleshooting the entire system at once.
Figure 6. CHEMCAD features pop up error messages and highlights in red Unit Operations with errors.
Improving Chemical Simulation Competency
As with any simulation program, the best way to improve your skills is to practice. Try out different simulations and change out the different reactor types to see how your simulation results vary using the different reactors.
Chemstations does offer training videos on their website, and there are many YouTube videos with different simulations available. Chemstations also offers in person training sessions, and they do have a variety of articles available, but it would be very helpful if CHEMCAD provided more training resources. They supply several example files with their CHEMCAD software, but more step-by-step guides would be useful to new users.