Thermodynamic basics for process modeling Basic guidance to help you avoid problems caused by selection of wrong thermodynamic model

Ivana Lukec
Thermodynamic basics for process modeling

The story about thermodynamics can hardly be a simple one. If it somehow could be a simple one and the world would be ideal – we would have only one nice equation that is good enough to describe any system. But in the unideal world, things are far from that. When developing a process model, a chemical engineer should have enough knowledge to be able to choose from a large number of thermodynamic systems. Chemical engineering as a field is progressing day by day and as a result of that the number of thermodynamic equations and parameters is increasing too with the aim to improve mathematical descriptions of different systems. With more complex thermodynamic systems, there comes a joint problem of more and more challenging mathematical operations. So, our knowledge becomes very important in selecting the most appropriate thermodynamics package. We will try to give some simple and practical instructions through this highly complex field of chemical engineering and process simulation.

Why do we need thermodynamics?

But, let's go from the beginning... why do we need to know thermodynamics at all to be able to perform a simulation?
Well, in some cases, we don't necessarily do... at least we don't necessarily have to know all the details, but we have to be aware of it. We don't need to know all that's behind in cases we are performing a simulation based on a large number of data sets and are using their relationships to build the model. In this case we are not looking into details of the system, only using the data to build the model. Examples are applications of artificial neural networks, linear regression etc. However, when talking about process simulation, most of the time we do refer to rigorous models and simulation tools, such as Hysys, Chemcad, Pro II etc. This approach is based on traditional chemical engineering laws and thermodynamics represent the essence of it. Therefore, when building a model in any process simulator, we need to make a selection of the proper thermodynamic system.
Some simple & practical instructions are defined to help you through the selection and minimize the possibility of problems.

One thing to have in mind: taking the wrong way while developing a process model can cause a huge waste of time and misleading results. So, try to be careful!

Thermodynamics finds its origin in experience and experiment, from which are formulated a few major postulates that form the foundation of the subject. Among those are 1st and the 2nd law of thermodynamics, the definition of enthalpy, entropy, equilibrium etc.
Selection of the appropriate thermodynamic package is one of the first steps when building the mathematical model. It is also one of the most important steps because a simple click of a mouse in most of the simulation programs will have the critical impact on simulation results. We might even not get the results. 
The choice of a thermodynamic package will have an impact on:

  • Accuracy of results,
  • Complexity of the calculation,
  • Convergence. 

What is the thermodynamics actually defining?

Thermodynamic packages consist of different sets of data and equations systems which represent a group of methods to perform all necessary thermodynamic calculations. Thermodynamic packages consist of all chemical and physical component properties together with different thermodynamic models which are applicable for different systems dependent on components and working conditions of the process (pressure, temperature). Most known are Soave-Redlich-Kwong, Peng-Robinson, Lee Kesler etc. It is our task to select one of them when building a model.

Also, it has to be noted that most applications require only one set, but complex flowsheets may be modeled best with several.

Step 1: Keeping the right way: overview of the process to be modeled

Overview of the modeled process refers to a review of the component list and expected working conditions: are components liquids or gasses, are they mostly hydrocarbons, is there any specific components such as H2S for example etc. When looking into temperature and pressure characteristics, it should be noted if the expected temperatures and pressures are around atmospheric or are they significantly higher. Some simulation programs may suggest what thermodynamic system suits best for a defined component list. However, it is always good to review the default selection. If there is no any suggestion or we don't want to follow it, then we should follow some general guidance based on our system definition and components list.

Step 2: Selecting the thermodynamic model

Some of the most important thermodynamic models are:

  • Peng-Robinson - a thermodynamic model ideal for vapour-liquid calculations as well as calculating liquid densities for hydrocarbon systems. Generally not useful for highly non-ideal systems.
  • Sour or modified Peng-Robinson – modification of Peng-Robinson model to extend its range of applicability to highly non-ideal systems.
  • Lee Kesler Plocker - a model is the most accurate general method for non-polar substances and mixtures. It is most often used for light hydrocarbons and for reformer systems containing high quantities of hydrogen.
  • Soave Redlich Kwong - in many cases, it provides comparable results to Peng-Robinson model, but its range of application is significantly more limited. It is most often used in gas and refining processes. Generally not useful for highly non-ideal systems.
  • Sour or modified Soave-Redlich-Kwong – modification of Soave-Redlich-Kwong model to extend its range of applicability to highly non-ideal systems.
  • NRTL – generally used for non-ideal liquid applications when calculating phase behavior (vapor-to-liquid or liquid-to-liquid equilibria).
  • UNIQUAC - generally used for non-ideal liquid applications when calculating phase behavior (vapor-to-liquid or liquid-to-liquid equilibrium).
  • Wilson - generally useful for slightly non-ideal applications.
  • Braun K10 - It is generally useful for heavy refinery hydrocarbons at low pressures.
  • Ideal - These methods should be used with pure component streams and streams with very similar components and for pressure around atmospheric pressure.

The table summarizes the priorities when choosing the thermodynamic model. The best selection is defined with "1", a little bit less appropriate but still possible with "2" and so on. Attention should be paid on operating pressure.

Step 3: Continue with model development

Upon the selection of the thermodynamic model, you can continue your work. In case you are facing any problems related to model accuracy or convergence, save your work, do another copy of your simulation and try to use another thermodynamic model for your simulation.
Selecting the proper thermodynamics can be a challenge many times, especially while modelling more complex process or a process with many different types of components. Use this information as a general guidance and in case of facing difficulties, you can refer to some of the following books: 

J.M. Smith, H.C. Van Ness, M.M. Abbott: Introduction to Chemical Engineering Thermodynamics

P. K. Nag: Basic and Applied Thermodynamics