Dr. Jim Silva
Chemical Engineer at GE
Drying Aqueous Salts for Polyetherimide
Monomer Synthesis
Basically what Dr. Silva was trying to
do was create a thermoplastic that could
retain its structural integrity up to
temperatures of 180 degrees Celsius. He wanted this plastic to be light weight,
as well as electro-platable which means it a very thin surface of metal can be
electro-chemically put onto it, giving it shiny finish without no corrosion. To make the polymer, he wanted
to make a bisphenol into an organic salt. The product needed to be created in water,
but because the rest of the process required water-free organic salt, the water
had to be removed. It was, however, fine
to have the product floating in a non-aqueous solvent.
When they did the process for one
bisphenol, it was well-behaved and resulted in fine crystals. The product was
full of water to start with, but they needed it to be dry. A
boiling solvent wasadded and then a chemical was sprayed into the solvent. This
caused evaporation and left behind the salt in the remaining solvent.
When this process was repeated with a
biphenol, it resulted in big particles and caking on the walls. It was
discovered that the new polymer was taking much longer to dry; therefore they
needed to make the new polymer at a much higher temperature. With this
approach, an extremely large amount of solvent would boil off with the water.
In order to fix this they changed from a
short path condenser to a partial reflux condenser that allowed much of the
condensate to return to the original mixture. The idea was to condense at a
temperature that was warm enough to keep the water if vapor phase but cool
enough to condense the solvent and return it to the mixture. But this process
is never perfect so some water inevitably gets condensed and returned to the
initial mixture. Because of this, they believed
that the partial reflux condenser would not work. They thought with the partial
reflux condenser too much water would be left behind in the mixture, but then
they did an experiment to see that wasn’t true. They recalled the Gibbs Phase
Rule which gave the theoretical reason for what they observed- that by fixing temperature
and pressure, the composition in the condenser would necessarily not change. As
a result they reduced the wasted solvent by an enormous amount, making the
process phenomenally more cost effective.
Terms
- · Thermoplastic- a material (usually resin based) that is rigid when cooled, but deformable when heated. The material can repeatedly be heated and cooled.
- · Cellulose –Repeating glucose units. Arguably the most common polymer on earth. Cannot be digested by humans. Starch (easily digestible by humans) is also a glucose polymer. However cellulose and starch link the repeating units in different ways, making a huge difference!
- · Bisphenol- a class of organic chemicals (organic meaning made from carbon, not meaning free of pesticide!) that is characterized by having two hydroxyl-phenyl groups. Saying something is a “bisphenol” is to categorize it chemically like saying something is a carbohydrate or an alcohol or an ester.
- · Biphenol – A subtly different substance from the one referred to as a bisphenol. Our guest used this to distinguish between two similar substances whose specific names he did not want to disclose.
- · Short path condenser – a condenser cools a vapor back into a fluid. A short path condenser removes that fluid from the original mixture. In our example, any solvent that is boiled off with the water gets removed from the process and then needs to be dealt with as recyclable or non-recyclable waste
- · Gibbs phase rule - essentially that degrees of freedom or things you can adjust in a process = number of phases minus number of components, plus two. So if T and P are fixed, the relative proportions of the mixture are defined, cannot change.
- · Partial reflux condenser –a condenser cools a vapor back into a fluid. A partial reflux condenser allows some of that condensate back into the initial mixture to be reboiled. Because the solvent and water have different boiling points, the two can be mostly separated using this process.
- · Ppm- parts per million. A term used to describe concentration, similar to percentage (parts per hundred)
- · Electroplating- coating something with a thin layer of metal through the use of electricity
- · Scale and scaling of a process- the larger the scale of the project, the more money and time it takes. More importantly, scaling a process is not as straightforward as it sounds. In this example doing something in the lab at the gram scale was simple, but doing it at the scale of metric tons posed many complications – large amounts of waste solvent, challenges to mixing and heating, etc. A classic example of scaling comes with the ratio of surface area to volume. Surface area increases at a squared rate, while volume increases at a cubed rate. In the case of mixing or heating, this has major implications. Imagine how long it takes to heat a 3 x 3 x 3 cube has a surface area of 54 square units and a volume of 27 cubic units. A 30 x 30 x 30 cube has a surface area of 5400 square units but a volume of 27,000 cubic units. You can easily see that heat transfer, fluid dynamics, and many other process variables will be affected by this dramatic shift in ratios.
Connections
·
Dr. Linhardt- The chemical engineering process and
scaling
· Dr. Ullal and Dr. Palermo-
Discussion of polymers
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