Unit operations that cannot be simulated

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Refinery Process Modeling by Gerald Kaes is a great book on using commercial process simulation packages to model refineries and related processes. But Kaes admits that 5 unit operations cannot be adequately simulated by typical commercial simulation software. I thought I’d share them with you, so you're alerted to these problem areas:

Background

First let us recall what process simulation programs can do well. Simulators perform heat and mass balances. They also use data on chemical species and thermodynamic methods to perform calculations which can estimate thermodynamic properties. For streams with only a few discrete chemical species this is usually no problem: they have been studied, and the program can look up all the coefficients and values to use in calculations to predict their properties. Assuming you set the program up properly, of course. Some methods will be the same ones you used in your thermo courses in school, and others will have added factors which are too annoying to do by hand but easily treated by a computer.

But you cannot look up component data for crude oil, which is full of literally thousands of complex molecules that are practically impossible to individually identify. Simulators get around this by creating “pseudo-components,” a slate of “fake” chemical species that together to try model the overall properties of the oil. The “pseudo-components” will have different boiling points, viscosities, etc., and the point is that by boiling, mixing, and combining these pseudo-components, you get an overall decent idea of how the oil streams in a refinery will act. When you distill the oil, your pseudo-components will also be distilled, and the disposition of the pseudo-components will try to predict the resulting product properties.

However, there are some chemical engineering problems where this whole approach falls down:

  • Asphalt processes (boiling points too high for open literature sources, no way to model some processes)
  • Lubricating oils (relies on aromatic chemistry and unusual solvents that cannot be modeled adequately)
  • Aromatic extraction (again, highly non-ideal chemistry that may not be covered in most simulators)
  • Chemical treatment processes, where an acid or base is used to “wash” away impurities like thiols/mercaptans, asphalts, odor, etc.
  • Diffusion/Adsorption processes like pressure swing adsorption

If you do have one of these processes, you’ll need special insider information to set up custom calculations to get around the problem. (Like the help of a technology vendor who sells the process and has loads of laboratory and operational experience). It's not impossible to model, but don't expect to do it out-of-the-box with your typical simulation program.

In your simulation, you may be able to use a cheap “hack” to work around the problem. For example, sometimes in preliminary simulations I will use simple splitters or “spreadsheet” operations to remove XX% of the H2 from a stream as a stand-in for my hydrogen pressure swing adsorber. This may let me get some rough working idea of what will happen, and the model can be improved with vendor data later on.

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3 Responses to Unit operations that cannot be simulated

  1. Tyras says:

    Can you please tell me more about the aromatics extraction simulation?

    I can`t find the book you quoted, and i could really use some information.
    I have to simulate the process, but i am new to this area, and i have been told that since it`s a non-ideal system, it is nearly impossible to simulate. The unit I am studying uses DEG as a solvent.
    I was going to use Hysys, can you recomend something else?

    Thank you

    • admin says:

      Tyras,

      Below, I will paraphrase for you the section of the book on aromatics. The book makes it sound like you need the help and/or data from your solvent vendor. If you need a second opinion, try contacting Aspentech Support or Eng-Tips.com. I’m afraid I never worked with DEG myself.

      Perhaps the vendor will give you the answers you need – either by modelling the process for you, or giving you the data you need to model it yourself. Often when I have to work with complicated vendor unit operations, I have to pay and trust the vendor to design their part, while I focus on modelling the rest of the plant surrounding their process.

      Paraphrase:

      “Aromatic extraction is commonly used to extract benzene, toluene, and xylene compounds from catalytic reformer products. There are several solvents such as furfural, tetra ethylene glycol (TETRA), sulfolane, and other polyglycols.

      Not all commercial simulators have distillation algorithms that can solve the highly non-ideal behaviour that occurs in aromatic extraction columns. Even when there is a satisfactory unit operation module in the simulator, there are other difficulties.

      The highly non-ideal phase behaviors must be represented with a liquid activity co-efficient method. The generalized methods of most simulators are inadequate. You usually need experimental data to derive accurate co-efficients. Such experimental data is usually proprietary and not available in open (free) literature.

      Most solvents mentioned above are patented and protected licensed processes. Therefore, a successful model for one of these processes generally involves a special agreement with the vendor to use proprietary data. Normally this is only given to licensees of the process.”

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