The Challenge of Unusual LLE

When friends and colleagues ask me what the CHEMCAD technical support group does, the best answer I have is: "We help chemical engineers address their challenges." This runs the gamut from simple to hard problems, and might include pushing for changes in CHEMCAD itself. One challenge we resolved was how to model some very non-ideal LLE systems in CHEMCAD.

The binary system of 1-propoxy 2-propanol and water has unusual behavior. There is both an upper and a lower critical solution temperature to the liquid-liquid region. Modeling this type of system in CHEMCAD proved to be quite a challenge.

What do we mean by upper and lower critical solution temperatures? As you increase temperature, you can form a second liquid phase. Keep increasing temperature, and the two liquid phases will coalesce back into a single liquid phase. A phase diagram for this behavior shows a closed ovoid.

If you wanted to model this, you would start by asking the following questions:
- Can CHEMCAD model liquid-liquid separation?
- Can CHEMCAD model this unusual liquid-liquid behavior?
- How do we (as users and engineers) inspect and adjust the simulator's model results for this problem?

You can use CHEMCAD to model a heterogeneous azeotrope—that is, a system where two liquid phases separate from each other. You can plot binary LLE diagrams, or the ternary 'binodal plot' that many of us remember from college.

For this simulation, we chose the NRTL K-value model. The NRTL model enables you to calculate γi from binary interaction parameters (BIPs). You can use γi to solve yi P = γi xi pisat. Normally this is used to calculate vapor-liquid equilibrium (Ki=yi/xi), but an optional modification to NRTL enables it to model liquid-liquid equilibrium. It's important to note that using NRTL with LLE will predict a second liquid phase only if the BIPs (from the CHEMCAD database or your work) will result in LLE.

It’s always wise to check what a K-value model is calculating, because sometimes you might need to adjust the settings for the model you’re using. One set of BIPs doesn't necessarily cover all conditions; the mathematical models aren't powerful enough to handle "all phenomena" with a single set of parameters.

In this case, using the Plot > Binary LLE and Plot > TPXY commands in CHEMCAD showed that the NRTL model did not match experimental data for the liquid-liquid separation.

To address this, we tried to quickly regress new BIPs from the literature data (Thermophysical > Regress BIPs). Regressing the LLE data did give us marginally better results, but we could not match the 'closed circle' seen in the experimental data for this unusual system.

Next, we decided to calculate the activity coefficient of each data point, and then regress a multi-parameter BIP from the activity coefficients. The results are shown below:


CHEMCAD now fits the experimental data quite well! We are now able to show the effects of having both an upper and a lower critical solution temperature. Using a sensitivity study and a flash unit makes it possible to move into (and back out of) the LLE region on a flowsheet.

Modeling Formaldehyde

Ever since my days on the support team, I’ve been fascinated by the formaldehyde-methanol-water system. Formaldehyde is one of the most important chemicals made; in 2005, worldwide production was approximately 21 million tons. It’s also highly reactive, and thus is usually handled in aqueous solution, sometimes with methanol as well to inhibit oxidation and polymerization reactions. The solution is usually referred to as formalin. This reactivity is what makes formaldehyde so interesting, valuable, and challenging to model.

Formaldehyde reacts with itself in water to form chains of poly-oxymethylene glycols, and with methanol to form chains of hemiformal. In the vapor, formaldehyde can react with water to form methylene glycol and with methanol to form hemiformal.

In a typical formalin solution, the bulk of formaldehyde is bound into methylene glycol (MG) and hemiformal (HF) molecules.

For a given solution of formaldehyde, methanol, and water, the following reactions apply:

Vapor:

CH₂O + H₂O =HO(CH₂O)H (MG)

CH₂O + CH₃OH = HO(CH₂O)CH₃ (HF)

Liquid:

CH₂O + H₂O =HO(CH₂O)H (MG)

HO(CH₂O)_nH + HO(CH₂O)H = HO(CH₂O)_n+1H+ H₂O

CH₂O + CH₃OH = HO(CH₂O)CH₃ (HF)

HO(CH₂O) _n CH₃ + HO(CH₂O)CH₃ = HO(CH₂O) _n+1 CH₃ + CH₃OH

Distribution of hemiformal concentrations as a function of formaldehyde concentration in methanol

MG_n and HF_n have very different vapor pressures than does pure formaldehyde, water, or methanol. It is not possible to determine the pure properties of these polymers directly, as they exist only in solution. The reactions also take longer to reach equilibrium than typical vapor-liquid equilibrium, which is one reason why much of the literature data is not thermodynamically consistent.

To model this behavior in CHEMCAD, we solve the reactions and thermodynamics together in the thermo model. By combining the equilibrium reactions with an activity coefficient model such as UNIFAC to predict the polymer activity coefficients, we can accurately predict the overall thermodynamic behavior of the system. We named our model the “Maurer” method based on the work of Professor Gerd Maurer and his group at Kaiserslautern University. Future CHEMCAD updates will include adaptations to the density and viscosity mixing models.

Welcome to the CHEMCAD Blog

As you’ve probably noticed, we have recently redesigned our web site and our company branding. This has been part of an ongoing effort to better reflect our core values—those things that make Chemstations who and what we are. We also wanted a way to communicate more directly to our customers about the work we do to constantly improve the CHEMCAD software.

This blog will serve as that direct communication link. Here you’ll get updates from Aaron Herrick and David Hill, two of the major forces behind CHEMCAD development and support. We often hear from customers that you’d like to know what they’re working on, and this seemed like an appropriate way to let you into their worlds. Of course, they also want your input! We hope you’ll comment here, or send them a note, a suggestion, encouragement, or whatever is on your mind.

Aaron will be actively looking for input for the development team. Look for posts to describe what challenges he’s facing, what input he needs from you, and potential new features or applications for CHEMCAD. David may post tips and tricks for new and advanced users, or tough support challenges his team has overcome. He’ll also be looking for input from you. What challenges are you facing in using CHEMCAD?

I’ll occasionally chime in myself. I’m most interested in understanding the changing process engineering landscape. We’ve built a nimble organization that has adapted to these changes over the past 21 years, and we want to continue that history of success.

I’d love to hear about other software tools you’re using in addition to CHEMCAD. What do you like? What do you dislike? I’m also interested in hearing about the pressures you’re facing: engineering challenges, business challenges, anything. The better we understand you, the better we can make CHEMCAD work for you.

So I invite you to stay tuned, and I hope you’ll become an active participant in the CHEMCAD dialogue.

To our existing and longtime customers: thank you for your business. We appreciate the opportunity to serve you.