Friday, April 08, 2011

Time to School the Chemists

There has been a lot of discussion the past few weeks in some of the chemistry blogs that may seem to be disjointed, but in my mind all have a common thread. Specifically, In the Pipeline talked about processes that he won't use in the lab (including UV reactions), ChemJobber talked about what is the maximum size scale-up is possible, and the Gaussling mentioned problems with excessively viscous liquids.

So what's the common thread? I look at these posts and I see chemists that are still practicing techniques that have been used for decades, and these techniques are all dominated by transport phenomena.

If you're a chemical engineer, you know all about transport phenomena and how to minimize its restraints, but if you're a chemist, you probably don't. Transport phenomena are the mechanisms that move heat, mass and motion around in your system. While mass diffusion, heat transfer and fluid dynamics may seem quite diverse, they aren't as in some cases all three can be described by a common mathematical formulation: flux = - (diffusivity coefficient) * (spatial gradient). The diffusivity coeffecient can be a diffusion coefficient, thermal conductivity, or viscosity, but that's not what I'm emphasizing today. Instead, the focus is on the spatial gradient. And the larger the samples dimensions are, the larger the gradient is, and the existence of the gradient is the problem since your sample is no longer homogeneous. And that's why scaling up is so difficult.

So what can be done about such a restrictive set of all-encompassing laws? Nothing of course; you can't violate them. What you can do is minimize the effects. Getting thin is the way to go. These laws are really ugly for big fat volumes, but pretty much fade away for thin samples. So throw away your beakers and flasks and replace them with something thin.

Consider polymer processing. With viscosities that are off the curve compared to normal organic solutions, you can never work with them in a fat flask, so instead you work with very thin films that are squished between the screw and the barrel of an extruder. Suddenly heat transfer is much easier, and the amount of material being pumped is less so that you don't need a motor the size of Hoover Dam's turbines.

Same with UV light reactions and coatings and ... There's a whole big huge world of chemistry out there effectively making products in the real world where the reactions pretty much ignore the laws of transport phenomena. And then all the problems noted above are reduced or eliminated.


Chemjobber said...

John, are you familiar with the current trend towards flow chemistry? Am I understanding it correctly that this is a way of getting around transport phenomena?

Anonymous said...

A lie begets a lie.