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.